best book(s) for middle game


You should study tactics at your level.your tactics rating here is very poor.game at this level are decided mainly by tactics.

IM Jeremy Silman's How to Reassess Your Chess is a very popular book which deals with a lot of middlegame topics. GM Yasser Seirawan has a book called Winning Chess Stategies which is fairly clearly written.

Simple Chess by Michael Stean.
https://www.amazon.com/Simple-Chess-New-Algebraic-Dover/dp/0486424200