Matt Charman, Oscar-nominated screenwriter of Stephen Spielberg's Bridge of Spies, and Mammoth Screen, producers of Poldark and Victoria, are producing a big budget TV series based on the book.

Maxwell Knight was a paradox. A jazz obsessive and nature enthusiast (he is the author of the definitive work on how to look after a gorilla), he is seen today as one of MI5's greatest spymasters, a man who did more than any other to break up British fascism during the Second World War - in spite of having once belonged to the British Fascisti himself. He was known to his agents and colleagues simply as M and was rumoured to be part of the inspiration for the character M in the James Bond series.

Knight became a legendary spymaster despite an almost total lack of qualifications. What set him apart from his peers was a mercurial ability to transform almost anyone into a fearless secret agent. He was the first in MI5 to grasp the potential of training female agents.

M is about more than just one man, however. In it, Hemming reveals for the first time the names and stories of seven men and women recruited by Knight on behalf of MI5 and then asked to infiltrate the most dangerous political organizations in Britain at that time. Until now their identities have been kept secret outside MI5. Drawn from every walk of life, they led double lives - often at great personal cost - in order to protect the country they loved. With the publication of this book, it will be possible at last to celebrate the lives of these courageous, selfless individuals.

Drawing on declassified documents, private family archives and interviews with retired MI5 officers as well as the families of MI5 agents, M reveals not just the shadowy world of espionage but a brilliant, enigmatic man at its centre.