I had vaguely remembered a book on this subject when I posted the above, but I couldn't think of the title. I remember now, it's The book of were-wolves by Sabine Baring-Gould, published in the mid-19th century. I've only just glanced through it, but I noticed that it has a long discussion of The Marechal de Retz, who I believe is the same person as Gilles de Rais. You can get a free on line copy from Google Books, and there is also a cheap Dover reprint available from the usual book sellers.

Jon Corelis
Kaleidoscope: Great Poems Set to Music