Middlegame books

What are some good middlegame books for my level? (I already own HTRYC 4th ed.)

Yes,

Also, be sure to check out...

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

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

Pachman, MCS, is what many of us used to get a big jump in understanding. Aside from that (and tactics books-If you haven't read 'The art of checkmate' I'd suggest starting there) I think best is not separating middle game from opening and playing thru annotated GM full games in the openings you like.

@SwimmerBill MCS isn't in algebraic notation. I'd love to get it, but I can't read it. (lol)

rjbue-- If you play d4, Sokolov has Winning Chess Middlegames that goes deep in d4 pawn structures-What it does cover is the best I've seen. Gelfer has a Dover book that looks excellent to me [based on what I can see]. Unfortunately, I'm 'experienced' (old) and the dover font & diagram sizes are too small for me to read  but that may not bother you. -Bill [By the way, descriptive is not that hard and if you learn it you can buy books from second hand bookstores too.]

I've looked at the table of contents in 'My System', and I feel it's really basic. Is my assumption correct? Is getting the book worth it? 

Perspective on Aron Nimzowitsch's "My System"...

https://www.chess.com/blog/RussBell/perspective-on-my-system-by-aron-nimzowitsch