Here's something for really hard core Agatha Christie fans: "Agatha Christie's Secret Notebooks. Fifty Years of Mysteries in the Making" by John Curran. (I haven't read it, just the review in the Washington Post.)
Apparently Christie used to jot down ideas for stories, plot strategies, etc. in a series of notebooks, interspersed with shopping lists, hairdresser appointments, and such stuff. Curran has sifted through this material and come up with a lot of good stuff about her thoughts about murder mechanisms and plot devices. From the review, it looks like the book will give away vital information about the plots of most of her books, but if you've already read them (I'm sure JoanK and I have read every mystery she's written) it would be great.