Okasaki: Purely Functional Data Structures (1996) [pdf]

The book by the same title and author (which is heavily based off of this paper) was actually one of the first books on functional programming I read. I don't think I would suggest it as an introductory text to anyone, but even so I really enjoyed reading it even though I didn't know what functional programming was at the time.

The book is about persistent data structures and what techniques can be used to measure their time complexity. I don't think there's much utility here for the software engineer (although I suppose there's probably some edge cases where it would come up), but as someone who's interested in programming language development I plan to keep this book around for a long time.