I would say Unnatural Death, by Dorothy L. Sayers, is my least favorite Wimsey book so far. That isn't saying much as I have really liked the previous books and this one was fine, just not as good a read as Murder Must Advertise or The Nine Tailors.
Wimsey and Parker are lunching when they are interrupted by a Doctor Carr who tells them about how being overly cautious had cost him his practice in a small town outside of London. Dr. Carr was treating an elderly lady with cancer who died far sooner than she should have. He suspected the niece must have done something but didn't know what, and the little bit of medical investigation he did was enough for people to gossip and stop coming to him.
So, we know who died, and who the killer must be, but not how or why. Wimsey has an interesting moral quandary as well: how responsible is he for the killer's actions after Wimsey begins investigating? We learn a bit more of Parker and meet Mrs. Climpson, a sort of Miss Marple character. She's Wimsey's female eyes and ears as everyone wants to gossip with little old ladies.
overall, a 5. It is quite good really; i guess i just didn't like the mystery being already solved at the start.