The Lion, the Witch and the Wardrobe by C.S. Lewis
Through the wardrobe lies the magical world of Narnia, held in perpetual winter by the White Witch. A few plucky British kids stumble into this realm, and are charged with bringing summer back to the frozen wasteland by some lion. Lewis' classic novel is actually book two in The Chronicles of Narnia, so if you want to get the full story, start with The Magician's Nephew.