Chris Okasaki's book "Purely Functional Data Structures" is not on Haskell per se, but it addresses data structures (and efficiency issues) from a higher level than the typical introductory FP book. All the data structures and code are available in Haskell in an appendix (and on-line). Amazon has 15 sample pages (hmm, must be a big seller). You should also look at the book "Algebra of Programming" by Richard Bird and Ooge de Moor: you will find much to satisfy your mathematical tastes, as well as motivations which include efficiency. Again the language is not exactly Haskell, but if past history is any guide, then Haskell will probably look more like the language of this book after a few years anyway :) . -- Fritz Ruehr PS: I found "A of P" on Barnes and Noble's site more easily than on Amazon.