In 1999, children's book author Julia Donaldson created a monster: "The Gruffalo," an award-winning story about a mouse who conjures a terrifying beast. With illustrations by Axel Scheffler, the book was a runaway bestseller. Now, the pair has released their third book in the series, "Gruffalo Granny," which has become the U.K.'s bestselling book of the year (of any genre), and the fastest-selling picture book ever. They talk with correspondent Elizabeth Palmer about their creative partnership, and about books that address big ideas for small people.