This is one of those not-actually-mysterious mysteries where the identity of the culprit is clear quite early on, to the detective as well as the reader, and the remainder of the story is the protagonist attempting to collect proof. Sometimes this works fine, especially if it is handled in a suspenseful way.
In this case even the police are convinced, but seem unable to think of any course of action. Campion is particularly ineffectual in this installment, perhaps a reaction on the author's part to swinging too far in the action-hero direction in the previous book. I wasn't convinced that the killer was really that diabolically clever and found the resolution disappointing and weak.
However, I did like the opening set-up with the artists and critics at the mildly Bohemian house party and the descriptions of the art. The supporting characters were a bit over the top but at least interesting.