I guess it depends on the world you wish to show them. the fairy books are poular but are writen by commity and are of poor story quality. My 8 year old loves Hank zipzer for its honesty and humour, Horrid henry because she laughs like a drain at the meaness. Animals of farthing wood, because it is about animals and she loves the adventure and the drama.
My 8 year old girl just started reading the Narnia series by C. S. Lewis. Other than that I've found her to be attracted to books which have something to laugh about (typically adults being very stupid).
As far as I know there are three books with Tiffany Aching as the main character. The Wee Free Men in which Tiffany defeats a fairy queen and rescues her brother (somebody else gets most of the credit for that though). Hat Full Of Sky in which Tiffany Aching starts learning to be a witch, mostly it's about a lot of hard work and not trying to cut corners with magic. Wintersmith is the third one and is about Tiffany getting mixed up in a dance with the seasons. The essence of Winter mistakes Tiffany for his partner, Summer and starts causing all sorts of trouble in an attempt to get her attention. All three are most enjoyable, I especially like the Macfeegle who were apparently thrown out of fairyland for being pished (the books glossary assures us this means very tired) all the time.