I strongly recommend you to buy a copy of
- Judgement and Planning in Chess by Max Euwe
It was recommended by my coach while I was an intermediate. It is definitely a classic work on middle game. The author provide analysis on frequent endgame positions that one must aim at achieving while he is still in the middle game.
Second book would be
- Attack and Defence by Mark Dvoretsky.
I am sure a player at your level must have heard of him, the star Russian Coach, many of his students are among the cream. This book, if you study seriously will equip you with various tools on how to think efficiently during a game (which is very important). There are numerous exercises which the author used to train his students.
Some chess trainers advocates studying as many positions as possible for a particular theme in the middle game. I recommend you to study these two books, going through each theme one at a time, carefully absorbing the ways to carry out a plan.