When Mary Pope Osborne wrote the first set of stories in the Magic Tree House series in 1992, she had a contract for four books, and she figured that would be it. But then she began getting letters from teachers, parents and kids.
"Those letters are priceless," she says. "I've memorized so many of them, like: 'Dear Mrs. Osborne, Your books almost made me smart!' or 'Dear Mrs. Osborne, I'm working on my own novel. ... It's not finished yet, it's scary, it's called The Septic System.'"