A Middlegame Lesson

"Think like a grandmaster" by Alexander Kotov concentrates on middlegame aspects. This book "feels" lightweight since it's very easy to read, yet it is also  conventionally called as "middlegame strategy lessons for dummies", because Kotov often quotes Steinitz (who pioneered the positional approach) yet explains his postulates in a way that's understandable by the average chess player, not only by grandmasters.