Possibilities for middlegame help:
https://www.chess.com/article/view/how-to-play-positional-chess
https://www.chess.com/article/view/test-your-positional-chess
https://www.chess.com/article/view/do-you-really-understand-positional-chess
https://www.chess.com/article/view/get-ready-to-test-your-positional-chess-again
https://www.chess.com/article/view/can-you-pass-this-positional-chess-test
https://www.chess.com/article/view/how-to-calculate-long-term-advantages-in-chess
https://www.chess.com/article/view/learning-basic-pawn-structures
https://www.chess.com/article/view/learning-pawn-structure-for-chess-players-under-2000
https://www.chess.com/article/view/your-pawn-structure-is-your-friend
https://www.chess.com/article/view/more-pawn-structures
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
Seirawan stuff:
http://seagaard.dk/review/eng/bo_beginner/ev_winning_chess.asp?KATID=BO&ID=BO-Beginner
http://www.nystar.com/tamarkin/review1.htm
50 Essential Chess Lessons by Steve Giddins
https://web.archive.org/web/20140708100833/http://www.chesscafe.com/text/review534.pdf
http://www.gambitbooks.com/pdfs/50_Essential_Chess_Lessons.pdf
Amateur's Mind by Jeremy Silman
https://web.archive.org/web/20140708094419/http://www.chesscafe.com/text/ammind.pdf
https://www.silmanjamespress.com/shop/chess/amateurs-mind-the-2nd-edition/
Simple Chess by Stean
https://web.archive.org/web/20140708104258/http://www.chesscafe.com/text/review400.pdf
http://store.doverpublications.com/0486424200.html
Chess Secrets: The Giants of Chess Strategy by Neil McDonald
https://web.archive.org/web/20140708092313/http://www.chesscafe.com/text/review620.pdf
Chess Strategy for Club Players by Herman Grooten
https://web.archive.org/web/20140708101926/http://www.chesscafe.com/text/review696.pdf
https://www.newinchess.com/media/wysiwyg/product_pdf/9042.pdf
Attacking Chess for Club Players by Herman Grooten
https://www.newinchess.com/media/wysiwyg/product_pdf/9032.pdf
Chess for Hawks
https://www.newinchess.com/media/wysiwyg/product_pdf/9041.pdf
Chess Strategy: Move by Move by Adam Hunt
https://web.archive.org/web/20140708093249/http://www.chesscafe.com/text/review890.pdf
The Power of Pawns by GM Jörg Hickl (2016)
https://chessbookreviews.wordpress.com/tag/the-power-of-pawns/
https://www.newinchess.com/media/wysiwyg/product_pdf/9034.pdf
Masters of the Chessboard by Richard Reti
http://www.thechessmind.net/blog/2012/4/1/book-notice-richard-retis-masters-of-the-chessboard.html
So basically I got to my current level through playing while reading tactics books, sheer luck and doing drills here and elsewhere. I'd read quite a few books on chess theory when I was a child, but they didn't cover the strategy of the chess masters that made their positions great, even being in a material disadvantage. I would like to improve very much, so if you got any advice on chess strategy or books that I could read to learn from the masters, I'd really appreciate that. For instance, I can understand stunts like the Marshall's Swindler or Die Weltminute von Waterloo, but fat chance I'd ever come up with something like that on my own.