The Lion, The Witch, and the Wardrobe (The Chronicles of Narnia #2) - C. S. Lewis

They open a door and enter a world

Narnia ... a land frozen in eternal winter ... a country waiting to be set free.

Four adventurers step through a wardrobe door and into the land of Narnia -- a land enslaved by the power of the White Witch. But when almost all hope is lost, the return of the Great Lion, Aslan, signals a great change ... and a great sacrifice.