By Narnian internal chronology, which is the first book?

By Narnian internal chronology, the first book is The Magician's Nephew. It tells how Aslan creates Narnia and how the world is first exposed to evil, long before the Pevensie children enter through the wardrobe.

The novel follows Digory Kirke and Polly Plummer, who travel between worlds using magical rings made through the experiments of Digory's Uncle Andrew. Their journey brings them to the dying world of Charn, where Digory awakens Jadis, the future White Witch. He later witnesses Aslan singing Narnia into existence and helps plant the tree that protects the new land from Jadis.

This answer applies specifically to the series' internal chronology, not its original publication order. The Lion, the Witch and the Wardrobe was published first in 1950, while The Magician's Nephew followed in 1955. Some readers still prefer publication order because it preserves the intended mystery of discovering Narnia through the wardrobe.