The Face of a Stranger (William Monk 1)

His name, they tell him, is William Monk, and he is a London police detective. But the accident that felled him has left him with only half a life; his memory and his entire past have vanished. As he tries to hide the truth, Monk returns to work and is assigned to investigate the brutal murder of a Crimean War hero and man about town. Which makes Monk's efforts doubly difficult, since he's forgotten his professional skills along with everything else...