There's also the series "for kids" that involves the witches and funny little elven dudes called the Nac Mac Feegle. Series starts with The Wee Free Men, then A Hat Full of Sky, Wintersmith, & I Shall Wear Midnight. They're marketed "for kids," but some of the most wonderful profound writing happens in them, and they didn't read altogether much differently than the "for adults" Discworld books.