amen zwa, esq.\(\textit{Purely Functional Data Structures}\) (1999) by Chris #Okasaki is a 💎.
All popular algorithms and data structures textbooks, including my favourite, the venerable #CLRS, are decidedly #imperative. The more a book leans toward the practical, the more blatantly imperative it gets.
But #Okasaki's book, the only one of its kind that I'm aware, is markedly different. It's functional—purely #functional. It's written in #StandardML, my favourite #programming language. It also includes #Haskell implementations in the appendix. The writing style is typical #CS—clear, concise, cogent.
I never got to teach from it, since it came out well after I escaped academia. But I heartily recommend serious CS students to study it, especially those with an unhealthy FP obsession. Experienced #FP programmers should read it, too. And FP novices must read it.