This is book 24 of the "Prey" series.

This note contains spoilers for the book.

Seems like the Davenport stories are continuing to be mostly outside of the Twin Cities. I guess there's more crime outstate.

Really major serial killer; they find an unused cistern with 21 skulls in it. And at least 4 people get killed after that.

Davenport's adopted daughter Letty gets to go to a crime scene with him, and makes some useful observations. She's about to head off to Stanford for college, but she seems committed to a career in law enforcement. I guess she's headed for a major federal agency, or perhaps intelligence work, rather than the local cops. I could see a series based on her!

And a nice twist (pardon the pun) with the long-dead partner.