user3149: I just finished reading My System. What's a good book to study afterwards for Expert Class? 

user9030: You will find tons of good attacking books for middlegame. 
What I want to recommend to you a defense gem. Defense is often a neglected part of chess study. And comparing attack materials, there are very little defense materials out there to study.
There is an excellent book for improving your defensive skills in middlegame positions, by GM Mihail Marin: http://www.gambitbooks.com/books/Secrets_of_Chess_Defence.html

user291: I highly recommend Johan Hellsten's Mastering Chess Strategy. It surveys pretty much all middlegame strategical considerations, is written very clearly, and has hundreds of exercises with detailed solutions. I definitely feel that a lot of what currently distinguishes me from a class A player are principles and patterns that I learned in this book. (His Mastering Endgame Strategy is also excellent.)

user16910: How to Reassess Your Chess, 4th Edition by Jeremy Silman is the book that has helped me progress the most. The author explains at length how to break down a position based on its imbalances with multiple examples from all level of plays.
The emphasis is given on general principles for positional play, not on critical lines or particular themes. In that regard, it's similar to the famous My System books from Nimzowitch. For example, high level games (but also amateur blitz games!) are analyzed under the light of certain principles, in an easy to understand way.
The book also tackles the subject of competitive mindset, what common pitfalls to avoid in reasoning, and conversely what kind of mindframe a player should build. That's a topic not often discussed in books, and has definitely helped me approach games more efficiently.
There are around 650 pages, with tests at the end of each chapter to confirm what was just studied, assorted with an Elo indication for each test's level, to gauge their difficulty. Silman's writing style is clear, direct, and most importantly enjoyable to read.
I definitely recommend this book if you're looking to strengthen your fundamentals, if you want to improve your mindset, or if you just want to see the game under a new light. It's suited (according to the author, and I'd agree) for players between 1400 Elo and 2200 Elo, so it should fit right into your goal of reaching Expert class (1800-2000).
To conclude, the worst point about this book in my opinion is how long of a read it is for a middlegame book, but every page is worth it.

user13012: Secrets of Modern Chess-Advances Since Nimzowitsch by John Watson

user3594: In addition to the other nice suggestions and to studying other player's games, I highly recommend simply solving middlegame-strategy problems (they are about finding the best strategy, not the best moves per se), and for that the book of "Chess Middlegame planning" by Peter Romanovsky makes for a perfect start. 
 [title "Example from the book: important strategic decision for black"]
 [fen "1rq3k1/5pb1/2p1r1p1/4p1Pp/1pP1P3/pP3Q1P/P1B2P2/2KR3R w - - 0 1"]

In the above diagram, white has just played the move c4, which is a big strategic mistake! How should black respond here? 

It might be tempting, say in a blitz game, to just push c5 here to protect the b pawn and lock the structure, but that would be a big miss! In this position, black has a very strong strategy at their disposal which requires the c5 square to be left free so the bishop can re-route. As Romanovsky goes about it, there are 4 stages to black's plan here: 1) Re-route the bishop to d4 (e.g. Bf8-Bc5-Bd4), 2) Consolidate the bishop with c5, 3) Open the f-file (which will be semi-open, that is, here only open for black after f6) 4) control the f-file with heavy pieces and invade white's structure via that file.

Once spotting this plan, you can tell that it's nearly impossible for white to muster a defense against what black intends to achieve, therefore, one can argue that white is objectively lost here after having played c4.
The book is filled with this kind of problems in the middlegame.

user8160: I would recommend Mark Dvoretsky's book 'Strategic Play' Olms. ISBN 978-3-283-00418-7.
It's a part of famous 'School Of Chess Excellence' series. (it's easy to find it online). I personally bought all four of them:
Endgame Analysis, 
Tactical Play, 
Strategic Play,
Opening Developments.
Mark Dvoretsky is considered one of the best chess trainers of all time, he raised a number of leading international grandmasters.  

3149: <p>I just finished reading My System. What's a good book to study afterwards for Expert Class? </p>
 9030: <p>You will find tons of good attacking books for middlegame. </p>

<p>What I want to recommend to you a defense gem. Defense is often a neglected part of chess study. And comparing attack materials, there are very little defense materials out there to study.</p>

<p>There is an excellent book for improving your defensive skills in middlegame positions, by GM Mihail Marin: <a href="http://www.gambitbooks.com/books/Secrets_of_Chess_Defence.html" rel="nofollow">http://www.gambitbooks.com/books/Secrets_of_Chess_Defence.html</a></p>
 291: <p>I highly recommend Johan Hellsten's <em>Mastering Chess Strategy</em>. It surveys pretty much all middlegame strategical considerations, is written very clearly, and has hundreds of exercises with detailed solutions. I definitely feel that a lot of what currently distinguishes me from a class A player are principles and patterns that I learned in this book. (His <em>Mastering Endgame Strategy</em> is also excellent.)</p>
 16910: <p><em>How to Reassess Your Chess, 4th Edition</em> by Jeremy Silman is the book that has helped me progress the most. The author explains at length how to break down a position based on its imbalances with multiple examples from all level of plays.</p>

<p>The emphasis is given on general principles for positional play, not on critical lines or particular themes. In that regard, it's similar to the famous <em>My System</em> books from Nimzowitch. For example, high level games (but also amateur blitz games!) are analyzed under the light of certain principles, in an easy to understand way.</p>

<p>The book also tackles the subject of competitive mindset, what common pitfalls to avoid in reasoning, and conversely what kind of mindframe a player should build. That's a topic not often discussed in books, and has definitely helped me approach games more efficiently.</p>

<p>There are around 650 pages, with tests at the end of each chapter to confirm what was just studied, assorted with an Elo indication for each test's level, to gauge their difficulty. Silman's writing style is clear, direct, and most importantly enjoyable to read.</p>

<p>I definitely recommend this book if you're looking to strengthen your fundamentals, if you want to improve your mindset, or if you just want to see the game under a new light. It's suited (according to the author, and I'd agree) for players between 1400 Elo and 2200 Elo, so it should fit right into your goal of reaching Expert class (1800-2000).</p>

<p>To conclude, the worst point about this book in my opinion is how long of a read it is for a middlegame book, but every page is worth it.</p>
 13012: <p>Secrets of Modern Chess-Advances Since Nimzowitsch by John Watson</p>
 3594: <p>In addition to the other nice suggestions and to studying other player's games, I highly recommend simply solving middlegame-strategy problems (they are about finding the best strategy, not the best moves per se), and for that the book of "<a href="https://www.scribd.com/document/108466014/Chess-Middlegame-Planning-Peter-Romanovsky" rel="nofollow noreferrer">Chess Middlegame planning</a>" by Peter Romanovsky makes for a perfect start. </p>

<pre><code> [title "Example from the book: important strategic decision for black"]
 [fen "1rq3k1/5pb1/2p1r1p1/4p1Pp/1pP1P3/pP3Q1P/P1B2P2/2KR3R w - - 0 1"]
</code></pre>

<p>In the above diagram, white has just played the move <code>c4,</code> which is a big strategic mistake! How should black respond here? </p>

<ul>
<li>It might be tempting, say in a blitz game, to just push <code>c5</code> here to protect the <code>b</code> pawn and lock the structure, but that would be a big miss! In this position, black has a very strong strategy at their disposal which requires the <code>c5</code> square to be left free so the bishop can re-route. As Romanovsky goes about it, there are 4 stages to black's plan here: 1) Re-route the bishop to <code>d4</code> (e.g. <code>Bf8-Bc5-Bd4</code>), 2) Consolidate the bishop with <code>c5,</code> 3) Open the f-file (which will be semi-open, that is, here only open for black after <code>f6</code>) 4) control the f-file with heavy pieces and invade white's structure via that file.</li>
</ul>

<p>Once spotting this plan, you can tell that it's nearly impossible for white to muster a defense against what black intends to achieve, therefore, one can argue that white is objectively lost here after having played <code>c4.</code>
The book is filled with this kind of problems in the middlegame.</p>
 8160: <p>I would recommend Mark Dvoretsky's book 'Strategic Play' Olms. ISBN 978-3-283-00418-7.
It's a part of famous 'School Of Chess Excellence' series. (it's easy to find it online). I personally bought all four of them:
Endgame Analysis, 
Tactical Play, 
Strategic Play,
Opening Developments.</p>

<p>Mark Dvoretsky is considered one of the best chess trainers of all time, he raised a number of leading international grandmasters.  </p>
