Clutch of Constables by Ngaio Marsh
First published in 1968.
My rating: 4 of 5 stars
I really liked the way this book was structured. Alleyn is lecturing a group of police academy students about the case that Troy got herself mixed up in. One of the main questions in this mystery is whether the other passengers on the boat are who they say they are and Marsh keeps the question open until the very end. There are a bunch of references to the painter Constable.