This is the sixth entry in Robert B. Parker's Sunny Randall series. Sunny is a female private investigator working in Boston. In this entry, she is working with her father, ex-cop Phil Randall. Twenty years previous, Phil failed to solve a string of murders committed by the "Spare Change Killer." Now it appears the killer is back to his old tricks; both Phil and Sunny are called in to play roles in the massive investigation.
Sunny has really evolved as a character. In early novels, she seemed more or less a female version of Spenser, the star of Parker's long-running major series of novels. But Sunny has become more introspective after going to see her shrink, Dr. Susan Silverman. There are ruminative monologues where Sunny mulls on her relationship with family and ex-hubby Richie; this is something that Spenser would not be caught dead doing. (Of course, now that I've said that, maybe he'll start up in the next book.)
Parker has started to play at (as they say in Ghostbusters) "crossing the streams" between his series; they are all set in the same universe, and characters from one series pop up in another. The last book had Jesse Stone play a major role. (And Sunny appeared in Jesse's previous book as well.) Here, in addition to Susan, long-time Spenser buddies Quirk, Belson, and Healy make an appearance. Is it only a matter of time before Sunny and Spenser himself meet? Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light. Sounds bad, but it could be interesting.
Anyway: a good entry in the series, with a thrilling (but unbelievable) climax.