The Button man : a Hugo Marston novel / Mark Pryor.

Enhanced descriptions from Syndetics:

In this prequel to The Bookseller, former FBI profiler Hugo Marston has just become head of security at the US Embassy in London. He's asked to protect a famous movie-star couple, Dayton Harper and Ginny Ferro, who, while filming a movie in rural England, killed a local man in a hit and run. The task turns from routine to disastrous almost immediately. Before Hugo even meets them, he finds out that Ferro has disappeared, and her body has been found hanging from an oak tree in a London cemetery. Hours later a distraught Harper gives Hugo the slip, and Hugo has no idea where he's run off to. Taking cues from a secretive young lady named Merlyn, and with a Member of Parliament along for the chase, Hugo's search leads to a quaint English village. There, instead of finding Harper, more bodies turn up. Teaming with local detectives and then venturing dangerously out on his own, Hugo struggles to find connections between the victims. Is this the work of a serial killer - or something else entirely? Knowing he's being tailed, the killer prepares for the final, public act of his murderous plan, and Hugo arrives just in time to play his part... .