What is one great, easy-to-read book about CS that is not widely known?

Data structures and algorithms: "Competitive Programmer’s Handbook" by Antti Laaksonen (https://cses.fi/book/book.pdf).

Operating systems: "Operating Systems: Three Easy Pieces" by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau (http://pages.cs.wisc.edu/~remzi/OSTEP/).