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.
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.
[1] http://hackage.haskell.org/packages/archive/prelude-safeenum... [2] 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.