As a mystery, it's a success. A crime is committed, Spenser is hired, the clues and suspects are introduced... By the end, the simple story grows into one of Parker's most complex plots.
And as social commentary, it also succeeds. In addition to Parker's comparison of different economic classes, he addresses the issue of homosexual prejudice. It's all effectively done without being excessively didactic.
Paper Doll is highly recommended for fans of the series, even if you don't follow the best approach of reading them in chronological sequence.
Since Spenser's favorite writer is Faulkner, there are several fun scenes when our hero in the Deep South (think "In the Heat of the Night"), dealing with the colorful local populace. Spenser heads South to talk to Olivia's father and has an interesting talk with the old man's servant by the name of Jefferson (which seems appropriately symbolic once the mystery is totally unraveled). We are also introduced to another possible continuing member of the supporting cast, Detective Lee Farrell, who is young and gay and rather surprised that Spenser does not care about either of those things as long as the guy can help him solve the case. The solid portrait of Farrell somewhat balances out the buffoonery of the Senator. Ultimately "Paper Doll" is another one of those Spenser novels where he is hired to do a job and the client tries to fire him but it is way to late to do any good. But then this is also one of those cases where trying to figure out the best thing to do is rather tricky, especially since Parker does not like our hero to ever use the same solution twice.