The Magician's Nephew
by C.S. Lewis
This is the sixth book of seven books in the Narnia series.
Before the white witch and before the wardrobe, a boy in England was playing with a neighbor girl. They stumble in on his Uncle, who has been experimenting with magic. Sending them off to another world. A forest filled with ponds each leading to its own world. Curiosity gets the better of them. As this story unfolds they see the death of a world and the birth of another. This prequel answer many mysteries of Narnia.
I loved this book. I think it is one of the more underrated books in the series. C. S. Lewis goes into great depth wrapping up loose ends here but I also love how he goes deeper into some of the ideas he has on people and the world around us. The story takes place more out of Narnia then in. Over all I feel that this was the most enjoyable.