I really like The Kingdom Keepers series by Ridley Pearson, but that's probably due to my love of everything Disney. But I'd totally recommend it if you like Walt Disney World because the books are based on the different parks. I don't really know how to explain it, but I'd sort of consider it like the secret world of Disney World at night because most things in the book happen during the night, and you never know when those It's A Small World dolls might come alive, right? ;)
I think I started reading the series when I was about 16 and I ended up spending more time on school and friends than reading. Now I'm trying to catch up because I really liked the first two books in the series (there's five books now).