In my opinion, pretty much anything by IM Jeremy Silman is pretty good.
- "How to Reassess Your Chess"
- "The Amateur's Mind"
Both of these books (by Silman) offer a good overview of fundamental positional ideas and concepts for intermediate players seeking to become more advanced. They both provide pretty much the same information overall, just the key difference is that "Reassess Your Chess" is a more in-depth version of Silman's overview of positional fundamentals, while "Amateur's Mind" is a more abridged version of that same overview.
Some other books I like:
- Silman's Complete Endgame Course (good for mastering all fundamental endgame concepts, ideas, and techniques)
- Silman's "Complete Book of Chess Strategy" (good for filling in any gaps in one's general chess knowledge to help reach advanced amateur level)
- Vukovic's "The Art of Attack In Chess" (good for improving attacking skills)
- Chernev's "Logical Chess: Move By Move" (good for improving one's skills in various aspects of the game)
If you’re under 1000 you don’t actually need to buy any book. YouTube videos are enough.