Fallen Angel: Purgatory

Cassiel Malcolm is one of the Fallen: Angels and Demons banished from their realm to ours. He is unable to lie and unable to die, and spends most of his time in a bottle.
When the daughter of an old friend asks for his help, Cassiel is thrown into a world of Fallen-hunting paramilitary groups, a Fallen Demon, and a pedophile-targeting serial killer. Some days you just want to climb back in to the bottle and close the lid...
Book 1 in an ongoing series.