Showing Rough Weather by Robert B. Parker

Heidi Bradshaw is wealthy, beautiful, and well-connected. She's also a notorious gold digger only recently separated from her latest husband - and she's hired Spenser to act as her stand-in spouse.

The Boston P.I. is to accompany Heidi to her private island to attend her daughter's wedding. It should be a straightforward job, but when his old nemesis Rugar - the Gray Man - arrives, Spenser realizes that something is amiss. As a storm strikes, cutting off the island, a kidnapping and a series of murders turn celebrations into chaos.

With six dead bodies and more questions than he can handle, Spenser begins a search for answers - and the Gray Man.