How to reases your chess by Jeremy Silman. If it is too high level for you, start with The Amateurs Mind, also by Silman.
Middlegame book suggestions
I don't believe you can do better than the two books by Euwe : Middlegame Book I and Middlegame Book II These are classics that have stood the test of time .
Reti's Masters of the Chessboard is an excellent and didactic introduction to the various ways you can conduct a game of chess. It features entertai ing games too !
''How to Reasses your chess'' by Silman. It is not exclusively about the middlegame but it teaches you how to recognize and exploit inbalances and will help you a lot. You will also learn the strengths and weaknesses of certain pieces in certain situations which helps you make a plan.
The Middle Game in Chess by Zonosko Borovosky
Not a monster text book (220 pages) the principles that he teaches are timeless and never go stale or out of date I read it when I was learning Chess 30 years ago and still have my origional copy well worn dog eared tanned and used Dover Books brought it back finally got a crisp new copy a few years ago.
"Another newer book which I liked very much is "Test your Positional Play" (Bellin & Ponzetto). Finally, you may like "Chess Strategy for Club Players" by Herman Grooten." Ha! That book was published at least 27 years ago! But I think it was reprinted a few years ago to appear new. Good book, though.
What is a good middlegame book? Not too hard please. My rating has sunk to about 1400. Thanks. Would appreciate if this Is on kindle due to eye problems.