That would be Donna Leon's Death at La Fenice. This little paperback must have come from Mary's bookshelf, but it has been laying around the house for a good while. I picked it up a couple of weeks ago, and just finished reading it. What a delight! The scenes that the author draws of the detective at home with his family alone make the book worth reading. (I read those scenes aloud to Carol, including the really funny one where the family plays Monopoly.) Throw in the location: Venice, where the American author lived for many years, and the challenging plot, which forced me to go back and make a careful list of the characters about 1/3 of the way through, and you have a delicious read.
Ms. Leon followed this one with 18 more in the series. Can't wait!