Book excerpt: As seen on pbs and Channel 4 documentary "Spying on Hitler's Army"...This is the story of the German émigrés who fled Hitler's regime and became secret listeners for British Intelligence during the Second World War. Behind the walls of the M Room (M for 'miked') they bugged the conversations of over 10,000 German PoWs, including 59 German Generals at Trent Park in North London. Providing a detailed, oft humorous, insight into life of the Generals in captivity, the book shows the farcical 'stage-set' in which they found themselves. But against this backdrop, the secret listeners eavesdropped on admission of war crimes and terrible atrocities against Russians, Poles and Jews; as well as details of an SS mutiny in a concentration camp in 1936, and Hitler's human 'stud farms'. This story places firmly on record just how much British and American Intelligence knew about Hitler's annihilation programme and how early. Why at the end of the war were these files not released for the war crimes trials to bring the perperators to justice? Was this one of the darkest secrets of the war? These transcripts, and thousands of others, of some of the most important Nazi secrets remained classified until 1999. During their clandestine work the secret listeners did not set eyes on a single German PoW, yet their work and the intelligence they gained was as significant for winning the war as Bletchley Park and cracking the Enigma Code. For over sixty years the listeners never spoke about their work, not even to their families. Many went to their grave bearing the secrets of the nation which had saved them from certain death.