This is book 36 of the "Spenser" series.

This note contains spoilers for the book.

Spenser and Hawk have to deal with The Gray Man again.

Spenser is hired for a support gig that makes little sense. He watches the Gray Man perform a complex and risky kidnapping and murder that makes little sense (6 murders, actually; plus Spenser kills one).

So he keeps his teeth into it, with the help of state and local police as well as Hawk this time, until he figures it out: the kidnap victim is actually the Gray Man's daughter, and there are complex connections in the past.