This is one of the very best computer science books I have ever read, and one that I find myself amazed by every time I pick it up. Better yet, he includes code for all his data structures in (almost) standard ML and (in an appendix) in haskell. He presents the (often arcane) material well and the gradual revelation of the concepts means that reading it I was continually amazed by the new dimensions he could add to familiar datastructures such as lists and trees.
Very deep and highly recommended for the serious functional programmer.