Middle Game
Learn the basic tactical themes. More or less these are:
fork / double attack
removing the defender / overworked piece
pin / skewer
discovered attack / discovered check
Then solve themed puzzles.
During games work hard not to lose any material, and look for opportunities to win your opponent's material with tactics.
Lots of possibilities for reading:
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
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
Simple Chess by Michael Stean
https://web.archive.org/web/20140708104258/http://www.chesscafe.com/text/review400.pdf
http://store.doverpublications.com/0486424200.html
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/
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
Understanding Chess Middlegames by GM John Nunn
https://web.archive.org/web/20140627012322/http://www.chesscafe.com/text/hansen154.pdf
http://www.gambitbooks.com/pdfs/Understanding_Chess_Middlegames.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
The Chess Attacker’s Handbook
https://www.newinchess.com/media/wysiwyg/product_pdf/7754.pdf
Chess Strategy: Move by Move by Adam Hunt
https://web.archive.org/web/20140708093249/http://www.chesscafe.com/text/review890.pdf
How to Reassess Your Chess (4th ed.) by Jeremy Silman
https://web.archive.org/web/20140708095832/http://www.chesscafe.com/text/review769.pdf
https://www.silmanjamespress.com/shop/chess/how-to-reassess-your-chess-4th-edition/
The Art of the Middlegame
http://dev.jeremysilman.com/shop/pc/Art-of-the-Middlegame-The-77p3554.htm
http://store.doverpublications.com/0486261549.html
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
Modern Chess Strategy by Pachman
http://store.doverpublications.com/0486202909.html
Zurich 1953 by Bronstein
http://exeterchessclub.org.uk/content/review-zurich-1953-bronstein
http://theweekinchess.com/john-watson-reviews/john-watson-book-review-106-zurich-1953-by-najdorf
http://store.doverpublications.com/0486238008.html