Death Behind Every Door

This used paperback book is well-loved on the outside, wonderful within. 

"Death Behind Every Door" by Heather Graham is a thriller set in Scotland, where FBI agents Luke Kendrick and Carly Guidry investigate a series of disappearances linked to a group emulating the notorious serial killer H.H. Holmes. The agents, part of the "Blackbird" unit of the Krewe of Hunters, uncover a society recreating Holmes's crimes and must race against time to stop them. The story involves historical details about Scotland, the use of ghosts to aid the investigation, and a dark, suspenseful atmosphere as the agents navigate the dangerous world of the serial killer society.