Best books to learn the "classical" approach for Black against the flank openings

I'd avoid Kmoch's book like the plague. It's an interesting attempt to label all sorts of positions using a new vocabulary, but just about nobody uses his terminology. There are some good ideas in the book, but you can find much more accessible books that cover the important material better. 

Stay away from Kmoch. 

I couldn't disagree more.  IMO, "Pawn Power In Chess" by Hans Kmoch is a great book - but not necessarily appropriate for the beginner-novice.  Every chess player has, from the beginning of their journey with the game, had to contend with learning unfamiliar concepts and terminology.  And some of the terminology introduced by Kmoch in his book has not made it into the familiar lexicon of chess.  However, exposure to and learning the unfamiliar is just a part of the process of learning chess (or anything in life for that matter).  So it is with some of the terminology introduced in Kmoch's book.  If one is put off by learning something in chess that is initially unfamiliar, (especially that which is designed to efficiently facilitate the discourse and the learning process), then perhaps chess is not the game for you.  Of course there are many other good chess books to learn similar concepts from.  Any good, instructive chess book can offer value in some way.  But the existence of other good books does not diminish the value of Kmoch's book.  And not everyone will appreciate the value of Kmoch's book in the same way.  But the same can be said of any chess book...

https://www.chess.com/blog/RussBell/pawn-power-in-chess-by-hans-kmoch

Russ, Kmoch reinvented the wheel and called it leukopenia. This isn't about understanding new concepts. It's about obfuscating key concepts with obtuse verbiage.

The point of Kmoch's terminology lies in efficient and succinct communication.  Its value is in facilitating discussion of concepts, techniques and positions which would otherwise take a much more lengthy phraseology to describe the same idea.  For example his introduction of the term 'leucopenia' is simply to avoid having to repeat the phrase "insufficient control of the light squares" over and over again in the discussion of this concept throughout the book.  This is the motivation behind his introduction of all of his "unusual" terminology.  For me, the terminology is simply an attempt at improving the efficiency of communication.  One should not be averse to learning a new vocabulary word if it can aid in understanding the ideas being communicated.  As we evolve in life, we are all exposed to a new and growing vocabulary as part of the process of learning anything.  Think of Kmoch's book as just another part of that evolutionary process - specific to chess.

"For example his introduction of the term 'leucopenia' is simply to avoid having to repeat the phrase "insufficient control of the light squares" over and over again in the discussion of this concept throughout the book."

Ahhhhhh.  That makes sense.  If I owned the book, and maybe I will one day, I would just make a glossary on one of the blank pages, and if I forget what a word means or represents, I would just flip to the glossary I created to look it up.  "Leucopenia" would definitely go on that glossary page, lol.

By the way.....

"leuko" is from the Greek for 'white'...

http://www.dictionary.com/browse/leuko-

and the suffix "penia" refers to 'lack of' or 'deficiency'....

https://www.thefreedictionary.com/-penia

A Hans Kmoch glossary...

https://www.chess.com/forum/view/chess-equipment/pawn-power-in-chess-by-hans-kmoch-glossary-of-terms

Understand that the essential value of Kmoch's book is not in his introduction of new or unusual terminology.  It is his presentation of fundamentally important concepts related to positional chess, especially when pawn play is a primary facet.  

So don't let exposure to a few new terms put you off from studying a great chess book.

Oh my goodness.  Someone already made the effort to help out the chess community.  That's terrific.  Thank you, Russ.

And you know how many people other than Kmoch used those terms? With very few exceptions, nobody. 

In fact, it occurs to me that I might just put together an expanded, more complete glossary of the terminology in Hans Kmoch's "Pawn Power In Chess".  It may not happen overnight, but I intend to do it.  If and when I do, I may publish it here first on the Chess.com forums and/or my blog.  Stay tuned.

That is largely true.  But don't let that stop you benefiting from the book!

Or maybe, just maybe, check out some of the other books that have been mentioned?

I would not advocate studying Hans Kmoch's book to the exclusion of any other chess book.  Or vice-versa...

Can we all agree that there is no reason to be concerned about reading Kmoch before looking at games in Sadler's QGD book?

"... Sadler ... directly aims this book at those players who don't have a lot of experience with the QGD. ... the nature of the book makes it of course more useful for players who are not yet so strong, ..."

https://web.archive.org/web/20140708234438/http://www.chesscafe.com/text/hansen15.txt

https://www.chess.com/blog/pfren/playing-1-d5-d5-a-classical-repertoire

I have made this point elsewhere in the forums, but I think it is worth repeating it here...

IMO it is not necessary to read books linearly. That is, restricting oneself to, and finishing one book before reading another book, or reading books in a particular order. In fact, I recommend a process of reading portions of several books on different topics at the same time, depending at any given instant on what you are motivated by and interested in. I believe that this approach would provide a more well-rounded exposure, over time, to the various topics.

An exception to this is where a given book might be too advanced for the player's skill level, i.e., in terms of the concepts that are presented, or how they are presented.  In that case it would be prudent to focus on the less "advanced" books, until such time as the player's skill becomes commensurate with that of the advanced book.  As an analogy - when embarking on the study of mathematics, attempting to study calculus before first mastering arithmetic and algebra would be an exercise in futility.

@RussBell, I get the strong impression that you believe Kmoch invented the concepts he describes with his jargon. He didn't. David Bronstein discussed the issue of light-squared weaknesses in some depth in his book on Zurich, 1953. And he wasn't even the first to do so. Blumenfeld also discussed light (and dark) squared weaknesses, as did Ragozin. 

The same is true for most of the terms that have some use for chess players. But Kmoch invented terms for concepts of such limited usefulness that they are not discussed by anyone else since. 

I agree, Kmoch has its uses. I agree, an advanced player may glean some useful information from Kmoch. But there are so many better books out there that aren't filled with jargon that it makes Kmoch's book just not worth the effort. 

An advanced player would be much better off going through Michael Stean's Simple Chess or Dvoretsky and Yusupov's works. A less advanced player would be much better off reading Watson's great books on chess strategy. 

But, as with all chess players, the book you read is better than the book you don't read. If you enjoy the jargon of Kmoch and learn from it, great. Just don't pretend that he has invented the wheel, or the concept of light squared weaknesses.

A person does not need to read Kmoch or Pachman before reading Sadler's QGD book.  Sadler's book seems to me to be a forerunner of Everyman's Move by Move books on openings.  I can't imagine Sadler's book not being helpful to anyone beginning to play the QGD.

Sadler's book is excellent, and easy to read.

@SmyslovFan -

You mischaracterize and distort what I have said in this forum regarding Kmoch's terminology. I have not asserted nor implied that he has invented any chess concepts.  My point, which I stated in my post #46 and which you have apparently chosen to ignore, is that Kmoch's  "jargon" is simply a means to efficiently describe concepts, techniques or positions.   Of course the jargon does not constitute invention of these elements of chess.

Now there is one that does not seem to have caught on.