I ordered the hardcover Proofs a few weeks ago after seeing the Calkin-Wilf tree [1] [2]. I cannot suggest it on the Kindle because you're going to want to writ

I ordered the hardcover Proofs a few weeks ago after seeing the Calkin-Wilf tree [1] [2]. I cannot suggest it on the Kindle because you're going to want to write in it—there are even large margins to encourage this kind of behavior. [1] http://hackage.haskell.org/packages/archive/prelude-safeenum/0.1.0.1/doc/html/Data-Number-CalkinWilf.html http://hackage.haskell.org/packages/archive/prelude-safeenum... [2] http://www.cs.ox.ac.uk/jeremy.gibbons/publications/rationals.pdf http://www.cs.ox.ac.uk/jeremy.gibbons/publications/rationals... It's one of the best books I've ever bought measured in raw joy and satisfaction per page. Like all proofs, reading is a very involved process, but these are genuinely stunning ones.