Related question: Should I make some sort of purist objection here? Nah, I guess not. If the Widow Parker is OK with it, I can go along for the ride. I always saw Spenser as Parker's alter ego, so it's a little odd to have the fictional guy live on.
As near as I can tell, Atkins decided to adopt/mimic Parker's writing style. That's not an easy thing to do, and it might not have been an easy choice to make. But, if I was presented, double-blind, excerpts from each author, I'd have a tough time distinguishing Atkins' writing from Parker's.
Nearly everyone's here: Susan, Hawk, Quirk, Belson, Henry Cimoli, Vinnie, Rita, … And Spenser remains his wisecracking self. Eventually, I caught some subtle distinctions: Atkins does the Boston geography thing to a significantly greater extent than Parker. And his Hawk seemed to be slightly more chatty than Parker's. But otherwise… dead on.
What about the plot? Spenser's client this time is 14-year-old, Mattie. Mattie is a foul-mouthed Southie girl whose mother was murdered four years previous. A lowlife thug was arrested and convicted for the deed, but Mattie is unconvinced; she saw two other thugs toss her mom into a car the night of the crime. But she couldn't, and can't, get the authorities to take her seriously.
Spenser is a sucker for this sort of thing, and (of course) it develops that Mattie is totally correct about the shoddy justice in the case. His technique is, as usual, to make himself a total pain in the wazoo to all involved, until the bad guys get rattled enough to try to thwart the investigation. That happens a lot here. It all builds up to an extended climax full of peril and action. Win!