user-1: I have been playing online chess (lichess.org) for three years at the rate of a few hours a week and I recently reached the score of 1850 Elo points in bullet mode.
So I want to take more seriously chess and in particular to learn the theory.
To tell you the truth, I've never done any tactical exercises, I don't know any openings or complex finals.
How do I go about it? I am not at all afraid to use books.

user12873: Learning theory involves some memorization of moves, but it's also important to understand the core ideas behind the moves. With these ideas, you can figure out what to play in novel situations not covered by the theory you've studied (or if you end up in a position that you forget).
There are different ways to study using an opening book. If you have a program like Chessbase, you could input the moves given, and also annotate in what you think the main ideas are. A good chess book will also take the time to explicitly explain the main ideas; some don't, which will require you to think on your own why a move was given. If you don't have Chessbase, I suppose you could go through the moves on an actual board, and jot down your notes somewhere (like in Word or a notepad).
If you're trying to get better at a new opening though, I'd suggest playing something longer than bullet - try at least 5 0. You want time to think about the position and apply the ideas (and moves) you've learned.

user804: The way I learned chess, was by playing over games from books by great players and going over the analysis they wrote between moves. I think I have over 500 chess books.
I played over all games of Kaprov, Fischer, Alekhine, Capablanca, Smyslov, tal, etc.. and tried to guess what the next move should be before looking at it. 
For learning how to think in chess, I found the book "Think Like a Grandmaster" by Alexander Kotov really good and taught me how to organize my chess thoughts during games and most important, how to be efficient in doing it. Sometimes we get our chess analysis cluttered during the game, and find we are going over the same analysis over and over. I am sure we all done this. This book helps in teaching one how to think better during the game (i.e. organize our thought process).
You also need to learn good chess openings. Learn few lines well. No need to learn very many lines. Just make sure to get to middle game in at least equal position where one then is out of the book.   Need also to learn basic end game techniques. I studied many books on end game. Rook ending, Pawn endings, and learn the basic basic patterns that can show up in end game.
But for me, the best way to get better in chess, was to go over the classic books written by the best players and read their own analysis of their games. 
Fischer's my best 60 games, and Alekhine 3 book collection of "My Best Games of Chess" written by him, and "My Best Games" By Kaprov, are such examples.  
This can be very time consuming also. So pick one player whose style you like, and try to study their games and do the analysis. 
A good collection with good analysis is the the chess informator books. I have large collection of these books. Study the games of the players you like their styles from these books.
At the end of the day, nothing beats playing chess with player who are stronger than you. After the game, go over it and see what mistakes you did. This is how you learn also.
My best US rating was 2430 I think or around there, and my ELO is 2310 (FM). I stopped going after the IM title as I got burned from chess. I do not play active chess any more. 
Just some thoughts. Hope they help.

-1: <p>I have been playing online chess (lichess.org) for three years at the rate of a few hours a week and I recently reached the score of 1850 Elo points in bullet mode.</p>

<p>So I want to take more seriously chess and in particular to learn the theory.
To tell you the truth, I've never done any tactical exercises, I don't know any openings or complex finals.</p>

<p>How do I go about it? I am not at all afraid to use books.</p>
 12873: <p>Learning theory involves some memorization of moves, but it's also important to understand the core ideas behind the moves. With these ideas, you can figure out what to play in novel situations not covered by the theory you've studied (or if you end up in a position that you forget).</p>

<p>There are different ways to study using an opening book. If you have a program like Chessbase, you could input the moves given, and also annotate in what you think the main ideas are. A good chess book will also take the time to explicitly explain the main ideas; some don't, which will require you to think on your own why a move was given. If you don't have Chessbase, I suppose you could go through the moves on an actual board, and jot down your notes somewhere (like in Word or a notepad).</p>

<p>If you're trying to get better at a new opening though, I'd suggest playing something longer than bullet - try at least 5 0. You want time to think about the position and apply the ideas (and moves) you've learned.</p>
 804: <p>The way I learned chess, was by playing over games from books by great players and going over the analysis they wrote between moves. I think I have over 500 chess books.</p>

<p>I played over all games of Kaprov, Fischer, Alekhine, Capablanca, Smyslov, tal, etc.. and tried to guess what the next move should be before looking at it. </p>

<p>For learning how to think in chess, I found the book "Think Like a Grandmaster" by Alexander Kotov really good and taught me how to organize my chess thoughts during games and most important, how to be efficient in doing it. Sometimes we get our chess analysis cluttered during the game, and find we are going over the same analysis over and over. I am sure we all done this. This book helps in teaching one how to think better during the game (i.e. organize our thought process).</p>

<p>You also need to learn good chess openings. Learn few lines well. No need to learn very many lines. Just make sure to get to middle game in at least equal position where one then is out of the book.   Need also to learn basic end game techniques. I studied many books on end game. Rook ending, Pawn endings, and learn the basic basic patterns that can show up in end game.</p>

<p>But for me, the best way to get better in chess, was to go over the classic books written by the best players and read their own analysis of their games. </p>

<p>Fischer's my best 60 games, and Alekhine 3 book collection of "My Best Games of Chess" written by him, and "My Best Games" By Kaprov, are such examples.  </p>

<p>This can be very time consuming also. So pick one player whose style you like, and try to study their games and do the analysis. </p>

<p>A good collection with good analysis is the the <code>chess informator</code> books. I have large collection of these books. Study the games of the players you like their styles from these books.</p>

<p>At the end of the day, nothing beats playing chess with player who are stronger than you. After the game, go over it and see what mistakes you did. This is how you learn also.</p>

<p>My best US rating was 2430 I think or around there, and my ELO is 2310 (FM). I stopped going after the IM title as I got burned from chess. I do not play active chess any more. </p>

<p>Just some thoughts. Hope they help.</p>
