Death behind every door : a novel / Heather Graham.

Enhanced descriptions from Syndetics:

In the shadows of a Scottish castle, something deadly is lurking in every corner...

When FBI special agent Luke Kendrick discovers chilling evidence of dozens of victims at a crime scene in London, he recognizes the calling card of the notorious H. H. Holmes Society. Named for the nineteenth-century monster regarded as America's first serial killer, the society is comprised of a twisted web of killers, working to carry out heinous murders in Holmes's memory. And now the group's been linked to a Scottish castle turned B and B--where guests check in but never check out.

Arriving at the sinister Graystone Castle posed as a tourist, Luke is joined by Special Agent Carly MacDonald, and the pair is immediately confronted by dangers hidden in the castle's depths. With guests disappearing and a network of killers closing in, Luke and Carly have only each other to rely on--and they'll have to race to cut off the head of the snake before the blood trail of the past can culminate in a very real and deadly present.

The Blackbird Files

Book 1: Death Behind Every Door
Book 2: Market for Murder
Book 3: Legacy of Blood