Which C.S. Lewis novel introduces Narnia through a wardrobe?

The C.S. Lewis novel that introduces Narnia through a wardrobe is The Lion, the Witch and the Wardrobe.

Published in 1950, the book follows the Pevensie siblings—Peter, Susan, Edmund, and Lucy—after they discover a passage into Narnia inside an old wardrobe. Lucy enters first and meets Mr. Tumnus, a faun, before the other children eventually join her.

The story presents Narnia under the rule of the White Witch, whose winter has lasted for a century. The return of Aslan changes the balance of power and leads to the restoration of the four siblings as rulers at Cair Paravel.

The Magician’s Nephew later explains Narnia’s creation and is first in the fictional chronology, but it was published sixth. That distinction causes a common mix-up between publication order and the internal timeline of the series.