The result has been the popular "Fablehaven" series, or what the author describes as "Jurassic Park meets Harry Potter." The fifth and final installment, "Fablehaven: Keys to the Demon Prison," arrives in stores Tuesday.
Beginning with book one in 2006, each has been a New York Times best seller — the series has sold more than 650,000 copies, according to Nielsen Bookscan — and Mull says a movie is now in the works.
Still, the series doesn't have the national profile that it could and certainly hasn't risen to the enormous success of other fantasy series, such as "Harry Potter" or "Twilight," Mull says.
"My agent likes to call it the best-selling series you've never heard of," he says.