For me it was reading Soltis' Pawn structure chess book, and Dvoretsky's Endgame Manual.
Endgames are an important part of middlegame strategy, because otherwise you don't know which piece trades and structures are favorable, a draw, or impossible.
And then the Soltis book is just a solid strategy book.
I don't really know how to improve these areas in my game and I feel I'm constantly making inaccuracies during the middle game. just wandering if any premium member has found a good coarse that helped them improve. I find that people give an example of a game showing you how a game was won with a fabulous outpost but as soon as you play a game with your new found knowledge your back at square one.