Blood Debt — Tanya Huff

THE HAUNTING
Henry Fitzroy, vampire, writer, and bastard son of Henry VIII, had survived for centuries by obeying the vampires' code. He did not slaughter needlessly, did not draw attention to himself, and never invaded another vampire's territory. But now Henry was about to do the unthinkable. He was going to break the code.

It began when Henry woke to the twilight--and the discovery that a ghost had invaded his inner sanctum. This was the start of a dangerous nightly game. Henry was allowed to ask one question of his mysterious visitor. If the answer was no, someone--innocent and unsuspecting--would die. It soon became clear that what this wraith--and the others who eventually joined it--wanted was revenge on those responsible for killing them.

Henry could not find the source of these murders on his own, nor could he ignore his unwanted guests. He had only one choice. To call private investigator Vicki Nelson and ask for help. Henry only hoped that he and Vicki would both survive the experience....