Something From The Nightside by Simon R. Green

John Taylor, a hard-boiled private investigator who works in a hidden, perpetual-night district of London where magic, monsters and vice are everyday hazards, takes what appears to be a routine case that soon unravels into a lethal tangle of demons, vampires, occult bargains and ancient conspiracies; navigating neon streets, corrupt gods and treacherous allies, he must rely on his murky talents and stubborn instincts to survive long enough to confront the Nightside’s darkest truths and the terrible price of dragging things back into the light.

Purchase from Bookshop.org