I am confident that it is in algebraic, but I think that the most recent revision (of one of the five) was about a decade ago. I suspect that there has not been a revision since then, because it is no longer seen as worthwhile to buy such books.
Encyclopedia of Chess Openings.

Informant has been algebraic since 1966. The first editions of ECO were written by world champions, but more recent editions tend to be done by lower level Serbian players, so it isn't quite as authoritative.
Nunn's Chess Openings came out in 1999. The 15th edition of Modern Chess Openings by Nick De Firmian came out in 2008. The Small Encyclopedia of Chess Openings came out in 2010. I don't think there has been a general encyclopedia released since then. If you want something more up-to-date than this, you could try monographs on individual openings, for example, the GM Repertoire series from Quality Chess. New in Chess Yearbook has articles on many specific lines. Chessbase and the Chess Informant periodical have annotated games in electronic formats.
Perhaps, Small Encyclopaedia of Chess Openings is the last book of this type that we will ever see.
https://web.archive.org/web/20140627063241/http://www.chesscafe.com/text/hansen145.pdf
If one really wants a detailed general opening reference, I think chessbase sells some sort of updated computer disk from time to time. I am not sure what the desired purpose is here, but I think the common perception is that it is no longer practical to publish a detailed reference in book form for all oenings. There are some not-so-detailed books that attempt to describe most openings (without tables of moves), but I think the last of those was about eight years ago.
https://web.archive.org/web/20140626173432/http://www.chesscafe.com/text/hansen128.pdf
http://www.gambitbooks.com/pdfs/FCO_Fundamental_Chess_Openings.pdf
If one is not too concerned with detail, perhaps it is not so important to have a recent update. Seirawan's Winning Chess Openings is not as comprehensive as FCO, but it is probably a lot more readable. If I remember correctly, it was written around 1999. Here is a review:
https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf
For a bunch of sample games, one could consider:
"... Nearly 170 pages [in The Mammoth Book of Chess] are ... devoted to various chess openings. ..." - Steve Goldberg (2010)
https://web.archive.org/web/20140708093123/http://www.chesscafe.com/text/review756.pdf
That sort of reading would be a pretty daunting task, but I think this book differs from most others because of its use of a lot of sample games.

Chessbase Magazine includes databases of recent games on specific lines with a short introductory text by players like Mihail Marin, Tibor Karolyi or lesser German masters. These mini-databases are included in Chessbase Opening Encyclopedia, but they are not really comprehensive nor all that easy to search.
I do not know much about chessbase stuff, but don't they sell some sort of nearly comprehensive collection of games? Is it possible to see what moves have been played in this or that position? Maybe one now has to go to the "cloud" for this sort of thing.

Chessbase Mega Database has a huge number of games. You can explore their opening tree, look at statistics for each move, or create your own opening keys. It is a bit different from an encyclopedia though as there are no GM evaluations for most lines. Some of the games are annotated by Carlsen, So, Kramnik et al. You kind of have to puzzle out which lines are better by yourself.

Thank you all for the useful info
It seems that there is a lot of books on certain openings (which would require a fortune to buy ). I have Watson's "Mastering Chess Openings" series which I found to have received much praise, still I was thinking about getting a more encyclopedic work.

Mohammad: If you want to buy ECO as a standard reference work, you'd probably be better off buying the first edition of the five volumes, written in the late 1970s and early 1980s. Yes, opening theory has moved on since then (quite a bit with some openings), but as Wayne pointed out above, nearly all the analysis in them was done by world class players at the time (Botvinnik, Geller, Korchnoi, Larsen, Hort, Portisch etc.) - the most recent editions are still worthwile, but the quality of analysis is often criticized (and not without reason). Then complement these with books on the openings you do play. I don't know which level you're at, but if you are beginner to intermediate, I would suggest Everyman Chess 'Starting Out' series (they will give you an excellent grounding). If you're intermediate to strong, Everyman Chess has several other excellent opening books, too. If you're a strong player then I would recommend checking out Quality Chess and their opening books. Thir books are aimed at very advanced and aimed at really strong players (their most well-known series is called 'Grandmaster Repertoire'). However, they're also full of didactic explanations which will serve amateurs as well. You just don't have to go through all analysis in one go. (It's great as reference after you've played some games in the opening.) You're already familiar with Watson's excellent books 'Mastering the Chess Openings' from the publisher Gambit. However, I've always found their other, specialized opening books too heavy on variations, and too short on explanation to be really useful.

Learn to understand pre-algebraic. It tells you more about the moves, because it tell you, for one thing eahat moved, and what side the board was on, and what the persoective of the mover was.
And a lot of great books were written using it.
You never know, it might even make a comeback.

i was gonna suggest buying used on ebay or similar... then looked myself... It would seem the 2nd hand price is about the same as the new price, possibly an indication they are starting to be collectable.
I wonder how much my set of 5 will be worth in 10 years time...
BTW, you'd probably be better off creating an electronic book, which is probably one of the reasons there hasn't been any recent volumes printed
... It seems that there is a lot of books on certain openings (which would require a fortune to buy ). I have Watson's "Mastering Chess Openings" series which I found to have received much praise, still I was thinking about getting a more encyclopedic work.
If money is a problem, my guess is that it is not going to be efficient to spend a lot of it on a multi-volume reference from decades ago. I do not know what openings you are considering, but it seems like a good bet that you can learn more about them, by spending the money on recent books on your chosen openings.
"... For new players, I cannot recommend books that use [an encyclopedic] type of presentation [of opening theory], because the explanatory prose that elaborates typical plans and ideas is usually absent, thus leaving the student without any clear idea why certain moves are played or even preferred over other apparently equivalent moves. ..." - FM Carsten Hansen (2010)

Some single volume, so-called encyclopedic openings reference books that might be of interest are...
FCO: Fundamental Chess Openings by Paul van der Sterren....an excellent relatively recent single volume openings reference...
https://www.amazon.com/FCO-Fundamental-Paul-Van-Sterren/dp/1906454132/ref=sr_1_1?s=books&ie=UTF8&qid=1496518423&sr=1-1&keywords=fundamental+chess+openings
Two older references which were, when published, considered encyclopedic and quite popular at the time are....
Nunn's Chess Openings...
https://www.amazon.com/Nunns-Chess-Openings-Cadogan-Books/dp/1857442210/ref=sr_1_1?s=books&ie=UTF8&qid=1496518238&sr=1-1&keywords=nunn%27s+chess+openings
and...
Modern Chess Openings by Nick DeFirmian...
https://www.amazon.com/Modern-Chess-Openings-15th-Firmian/dp/0812936825/ref=sr_1_1?s=books&ie=UTF8&qid=1496518590&sr=1-1&keywords=modern+chess+openings+15th+edition
Understand though, that it is not possible for these (or any) openings books to be truly encyclopedic, because 1) It is not practical for them to contain every variation that has ever been played for all openings 2) Opening theory is constantly changing and thus the latest most fashionable lines may not all be covered in these books.
Nevertheless, as books go, these are about as "encyclopedic" as it gets. And for someone who has not reached the level of "Master" or above, may be entirely adequate to their purposes, i.e., a survey exposure to a lot of openings, containing plenty of detail, particularly when it comes to mainlines which have stood the test of time...

FCO is not an encyclopedia. It is more of a primer. I do like NCO and MCO well enough, if you are looking for an encyclopedia with evaluations of a bunch of critical lines.

Watson's 'Mastering the Chess Openings' is excellent, and the same type of work as the van der Sterren's FCO (only, four volumes instead of one). Mohammad is looking for encyclopaedic works to flesh out his knowledge. Depending on his level, I would say that any one of ECO (and its condenced version Small ECO), MCO and NCO would serve him well for that purpose.
However, I think the old Russian coaching advice would also come in handy here. When you study openings, don't get bogged down with studying variations in minute detail, but focus on the general lines of each opening to ensure you develop a good understanding of it, both strategically and in terms of tactical themes and motifs. Secondly, look for 'role models'; very strong players who play your opening (we're talking someone with 200-300 games with an opening in say ChessBase Mega database) and score well with it. Study those games carefully, because those players have devoted a lot of time to the particular opening. For example, let's say that you want to play the Queen's Gambit Declined as Black. Then you'd do well to look up games played by Nigel Short, Rafael Vaganian, Smbat Lputian, Vishy Anand, etc. but also older generation players like Lasker, Capablanca, Rubinstein, Gideon Ståhlberg, Tigran Petrosian etc.

FCO is primarily an openings reference. At the same time it attempts to provide detailed commentary and teach something about the openings it covers - all the major openings.

FCO is not an opening's refference, it's an introduction.
It contains only the absolutely necessary lines and nothing more than that.
It's a great book for beginners but for anyone looking for a refference book FCO is not the book that can cover him.
Now we're debating semantics...

It's still semantics...
Any book which covers all the major openings and all the major variations of those openings, can be classified a reference...if it introduces, examines and explains, that doesn't disqualify it from being used as a reference...
I think the most useful thing is to get a clear idea of what Fundamental Chess Openings is like and how it differs from ECO, etc. If they work, these links may be helpful with that:
https://web.archive.org/web/20140626173432/http://www.chesscafe.com/text/hansen128.pdf
http://www.gambitbooks.com/pdfs/FCO_Fundamental_Chess_Openings.pdf
In case they do not work, I will add that FCO does not contain the tables of moves that one finds in ECO, etc. On the other hand, FCO has lots of sentences about the openings.
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Hello Everyone!
I'm thinking about buying ECO by Chessinformant, the one that appears in the link, however I don't know even if all its five volumes are in descriptive or algebric notation. Could you please send any photo of its inner pages? And do you know if there is a way to get it in Egypt? It seems that amazon doesn't ship it.
Thanks in advance.
http://www.chessinformant.org/eco-encyclopedia-of-chess-openings