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