Just remembered I've been sent a copy of "99 variations on a proof" by Philip Ording, to review.
It's brilliant, you should buy it if you've ever written, read, or thought about a proof.
press.princeton.edu/titles/13308.h…
In reply to @christianp
It's inventive, it's witty, and by god it's erudite. Every proof contains something new and interesting even to me, a jaded maths hipster.