The Folklore of Discworld by Terry Practchett and Jacqueline Simpson
This wasn't a Pratchett book in the strictest sense, more that it used Discworld characters to expand on the theme of folklore. For example, the final chapter is regarding Death, and it discusses how we arrived at the "typical" portrayal of him as a skeleton upon a white horse (who sadly is only called Binky on the Disc, and not on Earth). Chapters cover the major mythological creatures found on both the Disc and Earth (vampires, werewolves, elves etc) discussing various legends found on Earth, and how they are portrayed on the Disc. It also covers Witchcraft, standing stones and so on.
I liked this book a lot. I found it very interesting, and I will be hunting down some of the books mentioned in the bibliography. I feel Simpson has had more of an input in the book than Pratchett, and I was also a little disappointed that there was no mention of the "Death is female" belief that many have been made aware of through reading Neil Gaiman's Sandman series. That aside, this was a very enjoyable read, and it was a much easier read than the other Discworld crossover I've attempted, which was The Science of Discworld. Then again, I'm not very scientific, so that probably just says something about me!