Two books about the middlegame stage

In Book One: Static Features, Euwe and Kramer focus on pawn formations as fundamental to the strategies and tactics found during the middlegame stage of a game of Chess. They determine the relative worth of a chess piece, which is vital in determining how to value a potential exchange of pieces. And they look at open and closed formations; the struggle for open lines and weak Pawns.

In Book Two: Dynamic and Subjective Features, they focus on taking the initiative, during the middlegame stage, maneouvreing pieces for positional advantage; whether to 'liquidate' a piece to improve your situation; plus ways of attacking the opponent's king.