The Magician's Nephew by C. S. Lewis

Digory and Polly are playing in a tunnel above their neighborhood of houses when they open the wrong door and end up in Digory's Uncle Andrew's study. Suddenly, Digory and Polly find themselves in the ruins of Charn, where they meet a great and magical queen. But appearances aren't everything, and when Digory and Polly accidentally bring her back into their world, chaos reigns.

The Magician's Nephew comes before The Lion, the Witch and the Wardrobe in The Chronicles of Narnia. It tells of the beginning of Narnia, and how the White Witch came into it. This book has a light-hearted, almost casual kind of story, the kind that parents read to their children as a bedtime story.