I have Forward Chess. The technology is good. I wish they had more game collection books. I emailed them over a year ago, requesting more game collection books, like Alekhines two volume set, and Reshevskys book, and Botvinniks 100 games. They emailed me back asking the names of the publishers, etc. I figured, what the heck? They're in the business, and asking me to provide basic info on some of the most popular chess books of all time? All the chess publishers have a plethora of opening repertoire and strategy books, which, in itself isn't bad, but I don't care about endless opening repertoire books. I'm not planning on a match with Caruana any time soon. I have been playing for many years, and am just a below average club player. I don't care about being a master. I just enjoy occasionally immersing myself in the atmosphere of the past masters, and sometimes more modern players. So that's it, I would like to see more game collection books. I haven't even seen any of Karpovs game collection books in there either. So, in my mind, there are huge gaps in the selections currently available.
Forward Chess - Chess Studio - Chess Viewer - e+Chess Books - Chess King - Chessable

I have Forward Chess. The technology is good. I wish they had more game collection books...I just enjoy occasionally immersing myself in the atmosphere of the past masters, and sometimes more modern players. So that's it, I would like to see more game collection books. I haven't even seen any of Karpovs game collection books in there either.
I understand your position with respect to game collection books completely! I am in a quagmire currently, trying to decide to return game collection books that I recently purchased and buy the same titles in interactive book format, or should I keep the physical books to enjoy OTB and only get the latest improvement books in interactive format? I fear that I will be doing both over time.
Sadly I cannot recommend interactive books on or by Karpov myself. I read that the demand must be high to go through the work of converting the books.
Now if the books were published by Gambit Publications, it would be a different story. I have not seen Karpov books by Cadogan, Pergamon or even Quality Chess converted to interactive format.

there are huge gaps in the selections currently available.
To fill the current gaps in interactive chess book content, you should look into the "Chess Book Study". (I edited my post to include this title.) All you need is to own the PDF version of a game collections book that you love to read (and re-read). The app allows you to have ALL your books and magazines in your tablet and you can follow the annotations and the variations using the 2D chessboard. You will have to setup the board and move the pieces yourself since the board is not integrated to the data in the PDF.
Hopefully this will work for you until one of these app developers adds ALL the chess books into their library.

E+Chess doesn’t have this feature, which is idiotic, but the rest of the apps you mention do. Forward Chess has the biggest selection of books from the most publishers.

Agreed. I edited the topic post further to add these features to the "feature" list.
But what do you think about the VALUE of playing through the books using interactive 2D boards? What did you think of the asking PRICE for the books that you've purchased? From my estimation, most of the interactive books are listed at 40% off the MSRP of the physical books.

I just re-discovered "Chess Viewer" which offers Everyman Chess books exclusively. It, too, has an engine for the given position. Have you used that app before, and what do you think of it?
What a great topic! I was a total addict of chess books and since 4 years it has turned into an addiction to chess e-ebooks.
Forward Chess. I own 30+ books. Exists on all major platforms (Windows, iOS, Android) and a book purchased once can be used on any platform. Windows version is great because of large screen. "Copy FEN" feature is great as it allows you to copy/paste a position to a chess software of your choice and play or analyze. They reply promptly to bug reports or feature requests. My 2 main wishes: nicer board graphics and guess-the-move.
Gambit Chess Studio. I own 8 books. It's way behind Forward Chess in many things, you can't share books between iOS and Android, no Windows version. The best for Gambit would be to stop own software and to sell via Forward Chess or Chessable.
Chessable. I own 6 books (and many free ones). Excellent platform for drilling tactics and endgames (that's what I mostly use it for). Reply to emails very quickly. System of spaced repetitoons really motivates you to train regularly.
Chessbase. Books are published in CBH format by Chessbase, Everyman. Some books were converted from paper by enthusiasts. Text part is not as nice as on Forward chess but the board is much nicer.
Chess King courses. I have a full subscription which gives you access to some 60 courses on all 3 major platforms. iOS and Android apps are top level in terms of graphics and functionality. Web version lags behind. Many excellent beginner level courses, that I use to teach kids.

Forward Chess. I own 30+ books. Exists on all major platforms and a book purchased once can be used on any platform. "Copy FEN" feature is great as it allows you to copy/paste a position to a chess software of your choice and play or analyze. They reply promptly to bug reports or feature requests.
I just added "Chess Viewer" to the list of apps for interactive chess books. Have you had the chance to work on this tool that is exclusive to Everyman Chess publications? I agree that Everyman and Gambit should offer their titles on the apps of choice, the same way that people can now choose which streaming service they want to buy and watch their movies through.
Thank you for your experience and personal insight with the apps. I did not understand the "Copy FEN" feature until your explanation. Glad you appreciate the topic.
When I tried it few years ago Everyman Chess Viewer was horrible IMO. Fortunately we can read their books in CBH format using Fritz or Chessbase. And recently they started publishing with Forward Chess as well.

Chessbase. Books are published in CBH format by Chessbase, Everyman. Some books were converted from paper by enthusiasts. Text part is not as nice as on Forward chess but the board is much nicer.
Would you be able to explain the Chessbase interactive chess book option? Is there an app for tablets and smartphones? Are you referring to their FritzTrainer series? I would like to learn more about this. Thanks!
As for Chess King, I believe they have the largest collection of "guess the move" puzzles. Most of the Convekta titles revolved around that training model ever since its inception.
FOLLOW-UP:
I finally found a resource of hundreds of chess books were converted into CBH file format which allowed you to open the titles in the Chessbase program and follow through the games just like Chessable. I am unsure if the releases preserved the full text of the author's annotations because for me, that is what the value of a game collection chess book is measured against. Thanks for bringing this option to my attention. I will add it to the post topic immediately.
Chessbase. Books are published in CBH format by Chessbase, Everyman. Some books were converted from paper by enthusiasts. Text part is not as nice as on Forward chess but the board is much nicer.
Would you be able to explain the Chessbase interactive chess book option? Is there an app for tablets and smartphones? Are you referring to their FritzTrainer series? I would like to learn more about this. Thanks!
I don't own any FritzTrainer courses, but it looks like some of them are more similar to training databases while others are rather similar to books. I mostly use Chessbase/Fritz to read Everyman books in CBH format.
Unfortunately there is nothing for tablets and smartphones to read CBH. One can convert them to PGN but then text between the games is lost (text/comments within the games are preserved).

The Everyman viewer app has an engine, but it pales in comparison to the Forward Chess and Chess Studio apps because it does not allow the user to input his/her stupid moves on the 2D board. Without that feature, you can’t learn much of anything without negating the benefits of 2D learning in the first place (what’s the point of reading something on an e-chessboard if, when you become stumped, you have to manually input the position into your favorite engine anyway?
The ability to input one’s moves allows lower level players to play through more advanced books, because the user can test out the weak moves at his/her level that the FM/IM/GM author would never even remotely consider discussing due to limited space and the thought that such moves are so ridiculously weak that an amateur who doesn’t understand why ought to go back to more remedial chess learning.
The efficiency of 2D learning and the number of positions it allows one to get through (compared to 3D learning) is superior. Although, if all else is equal, then 3D learning is better.
For me and many others, our lives and schedules simply don’t give us a choice ... it is either 2D learning or quit.

Subjectively, Everyman Chess Viewer has an inferior interface. I have titles on each app, and it is just painful to stay focused when reading through the text on the side on Everyman Chess Viewer. It's as if the developers used XML technology to just drop whatever text came with the file to right window and fed the PGN data to the board on the left. Great for technical documentation but inconsiderate for book readers.
Scrolling through text and variations while following an automated chessboard is more efficient when done on one screen for us non-masters. This is especially true for game collections where the author has exhausted the "search for truth" such as Kasparov's My Great Predecessors and Kasparov on Kasparov books. And with the built-in engine on, we can see how the search for truths were drawn and dare to compare our own amateur ideas and have the engine evaluate it on the spot!
As for training, the best ones I've used are Chessable for new books and Chess King for Soviet School of Chess materials. Both have the "find the best move" quiz system and the systems track your progress and improvement.
Regardless, I still have a soft spot for playing the games on books over the board, especially exciting world championship games (Fischer-Spassky, Kasparov-Karpov, Anand-Carlsen) and the epic battles (Deep Blue-Kasparov, Second Piatigorsky Cup).
It is truly an exciting time to be a student of chess in 2019!
Did you see the latest sale on chessable.com? Are you getting any of them?
Sales and new books are tempting. The problem is that Chess King + Chessable + Forward Chess + Chessbase materials I own already are probably enough for 15 years of study. So I've made a resolution to not buy anything for 2-3 years.

It's now 2020. What's anyone's opinion on online chess books and resources nowadays?
My physical book library has remained almost static as I saw more quality chess books appear on chessable.com including the first three title of Kasparov's My Great Predecessors and Yusupov's chess training course.
ChessKing.com has added some new editions of their classics to library as well such as the latest iteration of CT-Art.
If there's ever a time to build up a library and to use idle time for chess study, being locked at home during this pandemic could be the best opportunity.

You can also try Chessvision.ai eBook Reader: https://ebook.chessvision.ai/ it makes chess PDFs interactive
I LOVE CHESS BOOKS. I truly appreciate the content of well-written chess books - from the excellent instruction for improvement in all stages of the game to the meaningful annotations to highlight the critical moments in game collections. From 2005 up until today, publishing houses have been producing new quality books than a non-professional player can read and absorb, especially when one prefers to use a physical chess board to go through the materials. GM Saptarshi Roy recently mentioned in a Chessbase India YouTube interview that he remembered, and enjoyed, the lessons from chess books better when he followed them using a physical board. Watch it here: https://youtu.be/0Ir5OOxOtz0
More importantly, technology has finally allowed us to interact with chess books using our tablets, smart phones and computers. These companies have converted the printed text into a medium that allows you to:
1. Read the original text by the book’s author
2. Use a two-dimensional chessboard to follow the mainlines AND the variations offered by the annotations
3. Activate a chess engine to evaluate any given position (not available for all apps)
4. Customize the color and pieces of the 2D chessboard (not available for all apps)
5. Use a 2D chessboard to solve the problems and puzzles in books that have them
6. Watch an embedded video of the author to further enrich the content such as the annotations or to enhance the learning experience (not available for all services)
7. Download the app for free and pay only for the content that you want
I would like to open a discussion on what people thought of the following chess apps and web services that delivered quality chess education to the faster, efficient and effective learning methods of 2019:
A. Forward Chess - offers the most chess by various publishers except Gambit Publications; free samples to try before you buy; chess engine option; purchases transferable to other platforms
B. Gambit Chess Studio - offers exclusive chess materials published by Gambit Publications only; chess engine option
C. Everyman Chess Viewer - offers exclusive chess materials published by Everyman Chess Publications only; chess engine option
D. E+Chess Books - one of the first book-to-app developers for smart phones and tablets; limited selection of chess books
E. Chess King - exclusively offers Convekta/Peshka materials which were prepared by actual trainers of the real Soviet School of Chess; free samples to try-before-you-buy; offers subscription to access all their content; purchases transferable across platforms (PC, OS X, Android, iOS); ideal for "find the best move" exercises and quizzes
F. Chessable.com - app in development; for now accessible via web browser on any computer, smart phone or tablet; library includes new chess books and independent contributors; ideal for "find the best move" exercises; recently acquired by PlayMagnus/Chess24
Special Mentions:
a. Chess Book Study - does not sell chess books; user provides the books and magazines in PDF file format; requires manual setup and manipulation of the chess pieces while reading the static pages
b. Chessbase Magazine - PC software; does not sell chess books but publishes own magazine; user must provide the books or Chessbase Magazine in CBH file format
c. Amazon Kindle - Hardware or app for phones and tablets; cheapest price for any ebook because the games are not interactive; will require physical chess set to follow the games or variations and analysis; Batsford publishing chess books are available
d. ChessVision.ai - Subscription-based web service that lets you upload your personal PDF files and have the diagrams converted to interactive 2D chess boards to have an online chess engine analyze every given position.
e. Chess PGN Master for Android - free app; opens chess games with annotations in .pgn file format and comes with Stockfish for analysis
So, what is your OPINION of these interactive chess book apps? Do you use them? Do you use or recommend ANOTHER app, which one and what do you like about it? Could you share an app that offers interactive chess books produced by MCFARLAND publications?
Are the PRICES of the interactive books fair? What is the VALUE of using interactive chess books compared to the print form to you - is it the PORTABILITY and CONSOLIDATION of your entire chess library into one device, the EFFICIENCY and EFFECTIVENESS for chess study, or even the REDUCTION of physical CLUTTER that you have to manage?
Lastly, in what capacity has this format IMPROVED your experience or enjoyment of chess? Did I just reveal your secret resource to the chess.com community?
Let’s discuss!
[EDITED to include contributions of the topic participants]