I noticed that most chess opening books only provide the correct variations for opening theory. However, what I don't understand is why most opening books don't explain the mistakes that one side could make and how you can capitalize on mistakes. I guess my issue is that most people deviate away from the theory, which means they are making some sort of a mistake. Are there any specific opening chess books that explain every single variation, including the mistakes that can be played in the opening by your opponent?
I noticed that when searching through a megadatabase that inferior moves or moves that are not part of theory are included in the opening. I just don't understand why when people publish chess opening books they don't show the opponent deviating from all the lines and playing inferior moves and explain why those are mistakes. For example, on the queen's gambit accepted, 1.d4 d5 2.c4 dxc4 3. e4, does any book show white how to properly play if black plays 3. b5? There are several ways black can keep trying to defend the c-pawn after 3. b5. Not sure if there is a book that covers this though. Another example, the king's gambit 1.e4 e5 2.f4 exf4 3.Nf3 and then 3.g5 could be played, which is not really any type of variation in the king's gambit and sort of considered a mistake on black's part with 3.g5. Again not sure if there is a book on the king's gambit that covers this line since most opening books don't cover mistakes made by the opponent.
Basically, I wanted to know if there are any chess opening books that cover the mistake variations in full detail or if this is something that you need to explore with an opening database weaker player's games until you fully build a comprehensive opening repertoire?
So, a couple things jump out.
First, most opening books I've ever seen DO cover mistakes, especially the common ones.
They typically cover them less than the main lines, of course, but they usually do cover them.
You must have just gotten really unlucky in your choice of opening books :)
Having said that, it does matter whether you're talking about a general reference that tries to cover all openings, or a book on a particular opening.
For the former, they just don't have the space to devote to all the mistakes, so they reasonably focus on the most critical main lines or the most common continuations.
For the latter, what I said initially applies. They almost always cover mistakes, especially if they're common (after all, it's much easier to make the case of Win With Opening X! if you can show wins against all these weak responses).
On your specific examples, the King's Gambit one is a bit odd.
1.e4 e5 2.f4 exf4 3.Nf3 g5 is not only a "type of variation", it's a main line (historically, you could argue it is THE main line; it is certainly the most often played).
I think it even has a decent claim to be the most testing line objectively in addition to being the most popular historically, although it's very close between 3...d6 and 3...g5
John Shaw's book on the King's Gambit spends the first 200+ pages analyzing 3...g5.
The QGA line you mention is indeed a somewhat unpopular line, and is almost definitely inferior to the main responses to 3.e4, but it's also not THAT bad, and has been ventured by some very strong players.
Looking at the table of contents for the first QGA book I could find online (Understanding the QGA, from Chess Stars), has a whole chapter (albeit a short one, about 12 pages) devoted to that response.
I'd guess you're probably looking at general reference books on the opening, and expecting too much from them (either that, or your just getting unlucky and picking crappy books).
Hope this helps!
EDIT:
I got really wrapped up in the comments about the specific lines, and forgot to actually respond to your question :)
I think you have a couple options when it comes to being prepared for suboptimal lines.
1) Prepare with decent books. As noted above, most decent books will cover the suboptimal lines that people are more likely to play.
2) Only look at openings as part of studying your own games, researching the lines your opponents play against you with all the resources at your disposal.
You have a pretty complete option here with the engine analysis, masters database, and lichess database.
That last especially is nice for this purpose because you can see what 2000 rated players try in blitz, for example, or other such things, to find lines that might be popular "mistakes".
I feel your approach towards openings and toward the game is questionable. You want some sort of book that gives you answers to anything that can happen so that you can memorize that book and you do not have to think ever. First such a book does not exist, as it would be extremely big. Second it would help you beat weaker players, but it would not help you play stronger players. Third it would make you think less instead of more, while thinking is the core of the game.
I don't understand how masters are able to remember opening lines and capitalize on the mistakes. Do they know that if someone deviates away from theory then they are making some sort of a mistake? Do they have this memorized or do they have to think about the position or sometimes both?
In my experience its a little bit of both. Learning openings is more than just memorizing lines. You need to understand why the moves are played. Getting an understanding of the positions that arises makes it easy to spot and take advantage of mistakes.
How do you go about "understanding positions"? Would you say that "understanding positions" has to do with calculation a majority of the time? I'm struggling getting the idea of how you "understand positions" in chess, especially the opening and transition into the middlegame.
Also, would a program such as chess position trainer (CPT) be useless in learning openings since it focuses on a flash-card concept? If so, what is the best way to remember the openings?
One very important part is understanding the pawn structures that arise and how to play them. F.ex. in the french you have the typical W:d4e5 B:e6d5 structure. This means that black light squared bishop is worthless. So one of blacks main ideas in the entire opening is to exchange it for whites bishop.
I find it useful to break openings down to their pawn structure to make them easier to understand.
CPT is far from useless. The training uses something similar to flashcards, yes, but it is great for organizing your repertoire, too, including taking notes, evaluating, etc. The note taking system comes with a reasonably full-featured text editor, including support for diagrams. Plus, with the way it handles transpositions, it is pretty much THE tool for openings.
Generally speaking you should know the idea behind a certain opening. If your opponent deviates from the main line it usualy makes it easier for you to execute your idea.
If there isn't any particular idea attached to your opening then you can develop normally. For example 1.e4 h6 of course you will go d4 and then develop knights etc, it is not very important in which order or will you play c4 Nc3 or Nc3 immediately, in both cases you have development advantage.
Let me give you an example of the first thing I've mentioned.
e4 c5 nf3 nc6 Bb5 g6 Bc6 dc6 d3 Bg7 h3 Nf6 a3
a3 is offbeat move, but it shouldn't distract you from your main goal and that is to play Nd7 Nf8 e5 Ne6 and establish a strng knight on d4.
This is the key in my opinion, if your opponent does something stupid your should, if possible, execute your plan or just develop normally.
Understanding positions isn't always about calculation as far as I can see (what do I know?) I see that some players are strategic in their understanding of position, which is not so much about calculation but about following general principles such as controlling the centre, not trapping a bad bishop behind its own pawns, not having knights on the edge of the board, trying to keep king or queen off same rank or file as opponents rooks/bishops to avoid discovery tactics, not having loose pieces or spectator pieces and not allowing the opponent to have more attacking pieces than you have defending pieces.
I guess by such an approach they are understanding weaknesses and strengths within a position without actively calculating, whereas some other players are all about tactical tricks and perhaps have less strategic approach and rely more on tactical calculations.
Maybe I came to the game too late, or maybe I am not blessed with much ability but when some players commentate their games you hear them talk about their opponents moves. With this advance they are trying to get a grip on this square but are weakening these squares...he has a good knight and I have a bad bishop so should trade, he has left a hole in his position that I should try to get this knight on etc such understanding allows exploitation of the position.
As for me I have trouble solving the CAPTCHA to post on here ^^