The Art of the Middle Game by Paul Keres and Alexander Kotov is a superb guide to this often neglected area of chess.
These two formidable grandmasters cover attacking the king, defense, importance of pawn structure, analysis, much more.
The Art of the Middle Game has an authoritative introduction by Harry Golombek. It also has Indexes of players and middle-game. It belongs in the library of every serious chess enthusiast.