This is book 6 of the "Stone Barrington" series.

This note contains spoilers for the book.

Vance Calder is murdered in LA, and Arrington calls Stone back from Italy—where he's half-way through marrying Dolce Bianchi (daughter of his friend the ex-mafia capo).

The LA cops are going for the easy answer. There's a series of follow-on murders, and they start looking at Stone for some of them, even.

Stone realizes that he's really still committed to Arrington, and was simply keeping from damaging her marriage previously. And Dolce turns out to be quite insane, murderously, when thwarted.

So, all in all, very complicated. Oh, and as I read it, it's clear at the end that Arrington did kill Vance. Stone doesn't seem to care, despite being somewhat upset about the previous lovers who turned out to be murderers.