I bought the original book (based on the Channel 4 TV series) after a visit to Bletchley Park in 2000 and enjoyed reading it. kharin has a slightly revised version, so whist most of this is a re-read, there are new sections, notably on the Pacific war effort.
There's an interesting introductory history of surveillance and intelligence eavesdropping within the British establishment from the time of Edward II. Whilst we may moan about today's surveillance society with CCTV, RFID, et al., in some sense there's nothing new under the sun. The description gets more detailed with the rise of the British Empire, and codebreaking activities during World War I.
It's always struck me as a bit of a grey area (because although I understand some of the principles, I certainly don't understand all the details) how much Enigma was an intrinsically flawed machine. The Polish cryptographers certainly laid some fundamental groundwork, as did the French. Without doubt, though, decryption was made easier by the way the German hierarchy arrogantly regarded it as unbreakable, and the way the lower ranks were lazy in choosing the necessary random settings. When there were technical changes, the consequences could have been severe: it was luck that most of the "Shark" blackout period coincided with a change in strategy by the German U-Boat fleet.
This is a light read and doesn't go into detail, but it nevertheless gives a good overview of the work that was done and the personalities involved. It certainly does nothing to dispel the picture of Bletchley as an Oxbridge out-station (conveniently situated mid-way between the two). I find it odd that there are extant photographs of Bletchley, including activity in the Huts, and Colossus, given Churchill's decree to destroy all evidence of Station X. It's perhaps even more curious that Robert Harris' Enigma features in the list of sources; to be fair, I haven't read the book, but the story behind the film - at the high level - is factually accurate (whilst the details are products of Harris' melodramatic imagination).