In this book, Terry Pratchett and his friend Jacqueline Simpson (who happens to be an experience folklorist,) get together and talk about the folklore that went into Discworld. They talk about how Sir Terry changed it, and what it is like in our world. There are chapters on each of the major races up until about Thud. For example the trolls. In that chapter they talk about how a lot of our world’s folklore has trolls turning to stone in the daylight and how on the discworld heat slows them down and makes their brains function less well. And also the differences, like how in our folklore trolls are close to human and often attracted to humans but on the discworld they are literally made of rock and interspecies romance is just not a thing.
Good for a non-fiction book, even though it wasn’t telling stories it was still talking about the shapes of stories. So that helped.