Donald E. Knuth — The Art Of Computer Programming
Context
A staple of CS literature. I've known of it for a while, until I had the immeasurable chance of finding it by accident on a book fair in 2022; Hardcover 1998 print, Volumes 1 - 4A, all for 80€!
Recap
This series is a concentrate of pure programming knowledge; practice and theory. This is not a "how to code" book, it contains very deep research on actual computational questions and problems with relevant answers and a ton of citations and exercises. That being said, YMMV depending on your edition.
Rating
If you're lucky enough to get your hands on these, don't hesitate. Even if you feel like you've wasted money because you rarely open them, you will find that they provide great material for meditation and thought, making you a better programmer long-term.