I also vote for "Secrets of Droon", by Tony Abbott. This series is about three kids who discover a magic staircase in their basement leading to another world full of magic, flying lizards, and fun, furry creatures. The kids make friends and go on various adventures.
My 8-yo daughter is devouring these books at a rate of about one a day -- each book is a few pages longer than the previous, helping the child build up the courage to tackle larger stories (Harry Potter can be daunting to a child who can't get past the idea of reading a 300+ page book!). We get the books from the local library because when she's done with this series, I expect there will be another one...