Imagine if the number 2 or 3 person at the CIA was a Soviet agent. Sounds impossible, right? Not so. Kim Philby was responsible for counter-intelligence for MI-6, the home of James Bond and England's equivalent to the CIA. Philby was a life-long communist agent, a mole in Britain's intelligence establishment.
The Master Spy is his biography - Knightly spent time with Philby in Russia, shortly before his death, and produced this work. It gives enormous insight into what might lead someone to betray their country and send many men to their deaths.