To Kill a Warlock by H.P. Mallory
Dulcie O’Neil is a fairy. And not the type to frolic in gardens. She’s a Regulator—a law-enforcement agent who monitors the creatures of the Netherworld to keep them from wreaking havoc in the mortal world.
When a warlock is murdered and Dulcie was the last person to see him alive, she must uncover the truth before she’s either deported back to the Netherworld, or she becomes the next victim.
Enter Knight Vander, a sinfully attractive investigator sent from the Netherworld to work the case with Dulcie.
Between battling her attraction to her self-appointed partner, keeping a sadomasochistic demon in check, and fending off the advances of a sexy and powerful vampire, Dulcie’s got her hands full.
As the body count increases, Dulcie finds herself battling dark magic, reconnoitering in S&M clubs and suffering the greatest of all betrayals.