[Haskell-cafe] Functional Programming Books

I was just putting together my Amazon wish list and was wondering if there are any great books on Haskell and/or functional programming that people think are must-reads. Okasaki's "Purely Functional Programming", Pierce's "Types and Programming Languages" are frequent recommendations. Smullyan's "To Mock a Mockingbird" seems like it might be a good example from the lighter end. Are there any other classics I should know about?