A Walk on the Nightside — Simon R. Green

Working in Nightside, a dark underworld hidden beneath the streets of London, John Taylor, a detective gifted with the ability to find things that are lost, searches for the missing daughter of a wealthy client, the Unholy Grail, and a missing singer whose voice has the power to cause death, in a trio of novels--Something from the Nightside, Agents of Light and Darkness, and Nightingale's Lament. Original.