Three months after Patrick Marnham published his excellent book on Jean Moulin, the Gaullist resistance leader betrayed in 1943 and arrested while at a meeting to unite the French underground, he received an anonymous letter, apparently unfinished. In admiring but also taunting tones, the writer, whom Marnham calls ‘the Ghost’, congratulated him on his good work but ticked him off for not investigating further. For all his sleuthing, Marnham had not quite established the identity of Moulin’s betrayer, a mystery that has obsessed France and that historians have tried and failed to solve since the end of the Second World War. With a little more ‘focus’, wrote the Ghost, as well as a ‘smidgen more imaginative reflection’, Marnham could have produced something remarkable.

There the matter might have rested, had Marnham not received a second communication. In a jiffy bag, along with various bits and pieces, came the end of the unfinished missive. The Ghost’s revelations included clues about a network called Prosper, based in the Sologne, three of whose members Marnham had lived with as a young man while learning French in the 1960s. And the fate of Prosper, the Ghost indicated, had links with that of Moulin. It was enough to send Marnham off on a new quest.

War in the Shadows is a book constructed in layers, each unfolding into the next. There is Marnham’s account of his stay in the Sologne, lovingly recalled, and his friendship with Pierre de Bernard, his wife, Anne-Marie, and her eldest daughter from her first marriage, Moune. All had worked as