How to study modern chess openings book?

At lower level you shouldn't memorize opening lines at all. When you get much stronger (for instance I am still too weak for that) and serious about chess, you start learning opening lines. There are multiple options for stronger players to do so. One of the most common is to buy Chessbase, an expensive piece of software that helps them analyze games and building opening repertoire. They can use reference books as well. 

Improving players should just stick to opening principles, pick a variation, memorize first 3-5 moves and go from there.

This is most of what we need to know about the opening stage of the game:

https://www.chess.com/blog/nklristic/surviving-the-opening-first-steps-to-chess-improvement

Like someone said, it is a reference book.  Going further and giving a comparison:

ECO, MCO, NCO, FCO.  Think of them as World Book Encyclopedia, Webster's Dictionary, Encyclopedia Britannica, and a Thesaurus.  Would you ever read any of those cover to cover?  Also, they are all outdated with this thing called the internet that is now between 25 and 30 years old.

Starting Out: The Alekhine, Dismantling the Sicilian, The Even More Flexible French, The Extreme Caro-Kann, Kings Indian Warfare, reading those 5 books would be like reading The Joy Luck Club,  Of Mice and Men, Pet Sematary, The Red Badge of Courage, and The Cat Who Ate Danish Modern, all books that one would read cover to cover and on vastly different topics and therefore would have vastly different audiences.

Hello Everyone,

These are some free resources, please see below:

https://www.chesspublishing.com/content/

https://lichess.org/video

https://www.365chess.com/opening.php

Best Regards.

Actually FCO is the exact opposite of a reference book, it has extremely clear verbal analysis, sparing in variations and diagrams every step of the way. 

OP if you're just looking up or curious about something you can simply read it by itself. If you want to do serious analysis of variations you will need to set up a board either in real life or on computer. My advice is to set up a real board first and go through it 99% that way first, then you can use computer analysis for specific questions you have that you can't answer. Playing opening moves and looking at the computer evaluations or database results after each one as some form of "truth" is rubbish, you should be concentrating on trying to understand the meaning and ideas and figuring out the justifications if you can.    

I put the lines into a PGN and put my own annotations with the various pawn breaks, dynamics, motifs, evaluation etc of the positions that arise from the lines and copy and paste the PGN in a word document to save

lol when I was young the only chess books i could find in bookstores (or my dad's collection) were game analysis books. and most of them were written by Fred Reinfeld. I never won a game against my dad. Then I found "Modern Chess Strategy" by Edward Lasker. Studying that book for a week took me from never winning a game to winning about 40% with him. I don't know his rank but he played tournaments in the 60's and was probably a slightly above average player.

My point is memorizing or analyzing games played by others taught me little. But learning to think strategically made a huge difference. Also spend half of the time you think about each move looking at the board from the opponents point of view. Try to get a sense of what they are trying to achieve. I'm not a great player, but i am a great deal better than I was before i learned how to learn chess.

The use of those books is like an encyclopedia: you look up things after you have played a game. You do not read cover to cover.