Posted: 11.02.2007 at 18.00
I'm currently reading Spy Catcher: The Candid Autobiography of a Senior Intelligence Officer. It's a fascinating book, to the extent where I'm staying up to 2am just to read it.
It deals largely with the Cambridge Five, including Kim Philby, and asserts that the fifth (never discovered) spy was Sir Roger Hollis, then head of MI5.