It's a series of books designed to introduce children to fantasy fiction - sort of a junior Thieves' World (in that it's a shared universe) with a touch of Ankh-Morpork, and it's doing very well, sales-wise. It's doing well in my library too, because I promote it. I have been reading my way through the series in hopes of getting one to write, and have managed to get hold of most of them. The series is a lot of fun and would be great fun to write. Children like fantasy, they enjoy sword-waving and this series works also for young adults, though not the kind who have read Lord of the Rings, but then it's not designed for them. Each story introduces more characters, buildings and such, which then become a part of the universe, but they're stand-alone, so none of this fat trilogy business where you have to read them all and read them in order.
That definitely meets with my approval! (I'm the one who suggested a panel on "The Dreaded Trilogy" at a Continuum and then found myself the only panel member who wasn't a trilogy writer, and we were facing a full-up hall with a lot of annoyed readers who HATE having to buy three books for one story. Hmm, maybe the subject for another post).