Clouds of Witness, by Dorothy Sayers
I picked this one up for a re-read when I saw it at work, because it's one I haven't read for some time. It's an early Wimsey mystery and not one of my favorites, but that still puts it very high on the list as far as mysteries in general go. (I forget now what I was actually looking for on the PR 6000 neighborhood--it wasn't Sayers.)
In Clouds of Witness we meet most of Lord Peter's family: Gerald, Duke of Denver, who is accused of murder, the Dowager Duchess--one of the best characters in the Wimsey world!--and as a bonus sister Mary meets her future husband too. There is fun with underground Socialists and one of the most horrible husbands around.
A very well-written mystery.