Try one of Jeremy Silman's books about mastering the endgame. The book is divided into sections depending on your rating.
The middle game is a strategy adjustment phase.
It's the end of your opening strategy and the beginning of your end game strategy.
During the middle game you can sac, sac and sac to reach an winning end game.
During the middle game, you need to see the horizon. What pieces will be left for the end game and where will the pawns be.
The sacrifices must be sound. The opening might be familiar, but the middle game normally holds the unknowns. The end game needs to be planned and it's planned during the middle game.
I like chess principles, so this is why I would pick a book like: Understanding Chess Middlegames by John Nunn
Whatever you do, avoid Eric Schiller.
@PixelatedParcel Yes.. the ratio of 1 book to each shelf is pretty important. Things really go wacky with the value of the whole equation when you deviate too heavily one way or the other.
Books won't be helpful to you yet. I looked at a few of your games and what you're lacking is board vision. Board Vision is the awareness of what all the pieces are doing and where they can and cannot safely move. The good news is that this is trainable. Tactics will be helpful. When you look at a position the first thing you should do is take the time to see which pieces (if any) are attacked or not defended. This will be tedious at first, but eventually with enough practice it will become subconscious and you'll stop making one or two move blunders that hang pieces outright
Books are helpful if you take the right ones and if you are able to read. All claiming this wont be the case prefer probably other media. Which is no sin. Take what is the best fitting learning material for you, giving you fun and strengthening your motivation.
For lower levels there's a bunch of good books. For systematic training the Stappenmethode is available in several languages meanwhile. This has been translated for good reasons. The Gambit Publishing books from Murray Chandler for kids are good learning material for adults too. US chessplayers know the Seirawan Winning ... series. Russian Chess House has a series starting with the books of Invanchenko.
There are more, high level material fitting to any taste. Some are explaining much with text, others give training material only. Look what your local vedors offer.
1.My system by Aaron nimzovitch
Here is an opinion of a grandmaster
#24 Chess Fundamentals by Capablanca is an excellent book. While it is tough to go thorough , Capablanca has adjusted it in 67 pages. If you try to read the book in 1 day then you will obviously fail. However if you read 2 pages per day then you shall be able to complete the book in 34 days.