I didn't find a discussion guide, but "Body in the library was "book of the month" here, and perhaps you can find some ideas there:http://community-archive.agathachristie.com/discussion/181/january-2014-book-of-the-month-the-body-in-the-librarySome of my own ideas that you may be able to use:For me, the most interesting aspect of the book (though it is a side issue in the book) is the relationships in the Conway family. The too-close-knit family, the son- and daughter-in-law who are a kind of replacement of the lost daughter and son, and the effect on them, and the way this moves the book (Adelaide's growing independence as the catalyist to Jefferson's attachment to Ruby). Also, perhaps, the similarity and differences between the two "Families" - the Jeffersons vs. Josie and Ruby - the former closeknit and affectionate, The latter alienated, but in both cases the dominant character is actually using the others to answer his/her own needs. And especially comparing the resolution of the two family issues.
I have just realised Craddock is not the main sleuth that is Miss Marple, It would have been nice if he had a book to himself like Battle did and Race and Ariadne although the last 2 weren't sleuth's
How about a Christmas pudding?