This is book 9 of the "Stone Barrington" series.

This note contains spoilers for the book.

This seems to be the beginning of the Herbie Fisher saga; Stone hires him to photograph an unfaithful husband in action, and Herbie manages to fall through the skylight. The cops think he killed the guy he lands on—but it turns out that the woman was an assassin planning to kill him, and he was dead by the time Herbie hit.

So, um, lots of complexities. Herbie is a complete idiot, and they have to actually haul him back from a Caribbean island at one point (and barely get him to court in time).

British Intelligence, and Stone's friend "Carpenter", are involved. They killed the assassin's parents, and the assassin is out to kill everybody involved with that operation, and gets the list down quite a lot in the course of this novel.