I just read his latest Spenser novel, Painted Ladies, and it is true that the master was not always at the top of his form in his later years. Sometimes the formula seemed a little... formulaic, and sometimes the story just didn't include enough development of the central characters to be interesting. The thing about Parker novels was that the mystery wasn't the point; whodunnit often was secondary to why, and the effect the case had on the detective was at least as important to the reader as the puzzle itself.
That said, Painted Ladies is one of the less successful Spenser novels. Hawk, Spenser's friend who sometimes helps out with extra muscle on dangerous cases, is nowhere to be found, mentioned only once to explain his absence. Susan Silverman, with whom many readers have legitimate issues, is central to the story (although she really doesn't bother me all that much). And the detective himself, who must be in his seventies by now, has not missed a step or lost a fistfight. Certain accommodations for a favorite character must be made.
Also, the mystery itself is fairly easy to solve, which isn't an enormous problem, but does weigh a little more heavily given that Spenser himself seems a little tired of the whole thing. He does what he does because he does it, but aside from that, he'd probably be just as happy spending a weekend cooking for Susan and, if it were the proper season (it's not, this time) watching a baseball game.
But what took me out of the plot on more than one occasion was an unusual factor--the character names. Usually something I wouldn't care about one way or another, in many cases the names here are either outdated or so unusual they draw attention to themselves, and that's the sign of an author trying to be too clever, or not caring enough.
I won't list them here, but suffice it to say some of the names, especially those of female characters, are howlers direct from a 1930s George Cukor movie. It starts with the client named Ashton Prince being blown to bits. Things go downhill from there.
As always, Painted Ladies represents a very readable Spenser novel. And by the time the next (last?) one, Sixkill, arrives next year, you'll probably forget the plot of this one. I've already forgotten the character names. But it took some work.