Which C.S. Lewis book explains how the world of Narnia was created?

The Magician's Nephew explains how the world of Narnia was created in C.S. Lewis's Chronicles of Narnia.

Published in 1955, the novel follows Digory Kirke and Polly Plummer as they travel between worlds using magical rings made by Digory's uncle Andrew. Their experiments take them to the dying world of Charn, where Digory awakens Jadis, and then to the darkness before Narnia's creation.

In Narnia, the children witness Aslan singing the new world into existence. Digory later brings back an apple from a tree planted in Narnia, and that apple becomes important to the story's conclusion. The book was published sixth among the seven Narnia novels but is commonly read first in chronological order because its events occur before those of the other stories. Digory also appears as Professor Kirke in The Lion, the Witch and the Wardrobe.