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I read Okasaki's PhD thesis by the same name, which he expanded into the book. Very well written, concise, full of useful theoretical information. The book expands into some data structures not covered in the thesis because the treatment would not have been original.

PFDS is a very good book even if you have no particular interest in functional programming. It's not the first book you should read on algorithms (hey, they didn't title it _Introduction to Algorithms_ for nothing), but I think PFDS is a very strong candidate as any one of the first four books you read on algorithms. It's short, it's not all that hard to understand, and the data structures and algorithms there can be relevant for reasons other than purely functional programming (especially: persistence).

After you read that, here are some papers on purely functional data structures suggested by the Theoretical CS stack exchange:

http://cstheory.stackexchange.com/questions/1539/whats-new-i...