The Attenbury Emeralds: The New Lord Peter Wimsey/Harriet Vane Mystery - Jill Paton Walsh

This is exactly like those tv episodes they put out now and then to save money and pad out a season or for a holiday or something - what are they called, clip shows? - where you've got a bunch of characters sitting around and reminiscing and the screen goes wavy and voila: there's a scene from an old episode. Only I haven't seen this tv series. I don't think I've ever seen a book presented like a clip show before.