Rivers of London (UK)
AKA
Midnight Riot (US)
Ben Aaronovitch
Gollancz, 2001
Rivers of London is an urban fantasy/mystery novel set, as you may have guessed, in London. The story follows Peter Grant, a young probationary constable on the London police force. He isn't actually a particularly good copper; he's competent, certainly, but he's more interested in history and architecture than keeping an eye out for rowdy drunks. Once his probationary period is up he is expecting (but certainly not wanting) to be assigned to a boring desk job. Then he meets the ghost.
The ghost is very spooky and all, but this is overshadowed by the fact that it was the witness to a recent murder, so Peter interviews it. The ghost has some actionable intel, which leads to a sticky problem... How do you report that your witness is dead, no one else can see him, and you yourself might not be able to contact him again? As it…