Disturbia. Thaumatology, #5

As Ceridwyn Brent and her half-succubus lover, Lily Carpenter, begin a training course to become Special Advisors to the Metropolitan Police Force's supernatural crimes division, the first of a series of mysterious deaths happens. A werewolf throws himself in front of a light-rail train, apparently being chased, though there is nothing visible following him. When another wolf from the same pack is found ripped apart in his home, the hunt for a supernatural killer begins.
The trail leads them to the darker side of London and into the underworld of illegal porn and snuff films which Lily is uncomfortably familiar with. Facing personal tragedy and Lily's ugly past, the couple must ultimately face a killer bent on revenge who cannot die.