The Magician's Nephew
Two children called Digory and Polly are exploring on a wet rainy day near the end of the 19th century when they find themselves in Digory's Uncle's study. They are tricked into heading out of the world and find themselves in a dying world called Charn. Due to Digory's curiosity Queen Jadis, the White Witch from The Lion, the Witch and the Wardrobe, comes on the scene and comes into England. A giantess and immensely strong, she wiped out her world with a word and tries to do the same thing on earth, but her magic has no power here. She starts a riot and they quickly try to get her out of the scene. They try to return her to Charn but accidentally take her to a new world, Narnia. Aslan then creates Narnia and then knowing of Jadis' presence sends Digory on a mission to get a tree that will protect Narnia for many years. Diggory does Aslan's bidding and plants the tree. In return Aslan gives him some fruit to bring home which cures Diggory's dying mother.  This book is the handoff to The Lion, the Witch and the Wardrobe in the chronology of the land of Narnia.