When Susan Macaulay (daughter of Francis and Edith Schaeffer, author of For the Children's Sake) was asked: "How do you define a living book?" she answered:
Oh, that's easy! That's one where they say: "One more chapter, Mommy, one more chapter!" That's a living book!
I think she nailed it. And no sweeter words have ever been heard by a homeschool mom. :)