Book excerpt: M is the name of an enigma. In his short and violent life, Michaelangelo Merisi, from Caravaggio, changed art for ever. In the process he laid bare his own sexual longing and the brutal realities of his life with shocking frankness. Like no painter before him and few since, M the man appears in his art. As a book about art and life and how they connect, there has never been anything quite like it.