The Magician's Nephew (TMN) is the first book in CS Lewis' Chronicles of Narnia. But it was the second book to be written (the first one was The Lion, The Witch and The Wardrobe). So you can say that TMN is a sort of prequel.
The nephew that the title refers to is Digory, the nephew of Uncle Andrew Ketterly, a demented recluse who thinks he is a magician. Although his parents are still alive, Digory is forced to live with his old maid aunt and bachelor uncle, Ms and Mr. Ketterly. While with the Ketterlys he befriends their neighbor's daughter, Polly Plummer. Together, Digory and Polly are sent into different worlds commencing with the brand-new world of Narnia. Along the way they experience various adventures, and meet Jadis the White Witch, who, unlike other white witches, is evil. They also meet Aslan, the majestic leader of the Narnians.