Prayers for Rain
The first "Kenzie/Gennaro" novel I read was the fourth book (Gone, Baby, Gone) which ended with the longtime friends and partners (and relatively new lovers) splitting up. I then went back and caught up on the characters back story with the first three books. Prayers for Rain is the fifth book, and deals with some of the aftermath of Gone, Baby, Gone. Ptraick Kenzie, missing Angie Gennaro desperately, has lost his enthusiasm for the PI game. He begins to investigate the apparently suicide of former client Karen Nichols (who reached out once to him when he was "too busy" to return her call), though no one seems interested (including her mother and step father). After finding a few oddities, he gets Gennaro (now working for a big security firm) to help out some on the side.
They find what appears to be a cold blooded, brilliant mastermind, who is manipulating those he meets by finding out their weak points. Is this the missing step brother of Karen Nichols? How does he know so much information about his victims? Angie and Patrick, along with help from their fearsome friend Buba Rogowski, decide that they can't let the case go.
As with all of their cases, there ends up being gunfire, with a final shootout at the villain's underground lair. We do get a full Gennaro/Kenzie reunion, as they both realize that the other may have been right before. There's a family reunion with some happiness, but also a coda when we find out that we might not have know who the real villain was.