What is the best book to understand middlegame planning

These two are excellent.

https://www.amazon.com/Pawn-Structure-Chess-Andrew-Soltis/dp/1849940703

https://www.amazon.com/Inner-Game-Chess-How-Calculate/dp/0812922913

Thanks

I would add “Chess Structures: a Grandmaster Guide” by Mauricio Flores Rios. It’s a good companion to Soltis and somewhat easier to follow, because it lays out the material in a more organized fashion. There’s some overlap, but you learn something from each book even for structures they both cover.

The books mentioned above are very good.

If you haven't yet read "Simple Chess" by Michael Stean, I would highly recommend it.  It is a primer of positional chess concepts and the implementation of plans and strategies that arise in practice....

https://www.amazon.com/Simple-Chess-New-Algebraic-Dover/dp/0486424200/ref=sr_1_1?s=books&ie=UTF8&qid=1515368586&sr=1-1&keywords=simple+chess+stean

For more books dealing with these topics...

Good Positional Chess, Planning & Strategy Books for Beginners and Beyond...

https://www.chess.com/blog/RussBell/introduction-to-positional-chess-planning-strategy