I was going to recommend the Hodges biography of Alan Turing, but it appears to be (sadly) out of print.<p><a href="http://www.amazon.com/Alan-Turing-Enigma-Andrew-Hodges/dp/0802775802/ref=sr_1_1?ie=UTF8&#38;s=books&#38;qid=1263905638&#38;sr=8-1" rel="nofollow">http://www.amazon.com/Alan-Turing-Enigma-Andrew-Hodges/dp/08...</a><p>[edit: In the US.  Someone else posted the link to the UK edition, thanks!]