Purely Functional Data Structures by Chris Okasaki

If you're looking to dive deep into computer science, this is a classic text on functional programming. It's Chris Okasaki's Purely Functional Data Structures, which is pretty much the go-to resource for understanding how to implement data structures in languages like Haskell or Standard ML. The cover has that recognizable geometric pattern and it's in good shape. It's a great find for any developer or student wanting to level up their algorithmic thinking. Used, but well-kept.