[mtrimpe]

I agree that it's a great book; but I wouldn't recommend it as an entry point into understanding purely functional data structures.

Okasaki fleshes out a couple of examples and explains his thought processes and heuristics for developing such data structures quite well; but they're very much still notes on the early-stage exploration of the concept.

From a historical perspective it's still a fascinating read though and definitely recommend it if you want to read up on the origins of immutable data structures.