From the Goodreads Dennis Lehane interview:
GoodReads: Do you have any plans to return to Kenzie and Gennaro? What’s next?
Dennis Lehane: What’s next is the sixth Kenzie-Gennaro book. Really.
Okay Dennis. Let’s do it. A new Kenzie-Gennaro book in 2010! Or perhaps 2011. Take your time. I want it to be good.