Right now I'm trying to focus my studies on the middle game. Currently I am studying Ludek Pachman's "The Middle Game in Chess." Can anyone recommend some other good books that focus solely on the middle game? I would be eternally grateful. Thanks. 

Bronstein's "Zurich 1953" tournament book is a classic which focuses mainly on the Middlegame.

Pachman's "Modern Chess Strategy"? or Fine's "The Middle Game in Chess" ?-- I don't think pachman wrote a 'middle game in chess' did he?

"How to Reassess Your Chess" Jeremy Silman

"The Middlegame" vols 1 and 2 Euwe and Kramer

"Simple Chess" Michael Stean

"My System" Nimzowitsch