Middlegame Secret from some books

The most important thing about playing the middlegame is to have a comprehensive plan. This plan must evolve from a correct evaluation of the position reached following the opening. If the opening has been reasonably well handled by both sides, one must then recall what the prescribed plan(s) for the respective position(s) resulting from that opening is and judge whether or not it can be modified to suit the situation on the board. While you would have to eventually read the classic "My Sytem" by Nimzowitsch, you should also read "Simple Chess" by Michael Stean and you'll discover the richness of the middlegame and how correct plans are chosen to fit the position on the board.