Which Narnia book by C.S. Lewis introduces the wardrobe portal to the fantasy world?

The Lion, the Witch and the Wardrobe introduces C.S. Lewis's wardrobe portal to Narnia. The novel was published in 1950 and follows Peter, Susan, Edmund, and Lucy Pevensie.

Lucy is the first of the siblings to enter Narnia, discovering the world through a wardrobe in a country house. The story features the White Witch's winter, the lion Aslan, and the children's role in Narnia's conflict.

The book was the first Narnia novel published, although some reading orders place The Magician's Nephew first because it tells Narnia's creation. That difference concerns sequence, not the identity of the book that introduces the wardrobe.