The Lion, the Witch and the Wardrobe – 85 million
C.S. Lewis published this religious-themed fantasy novel in 1950. Intended as a children's book, it's set during World War II and is a classic example of a "magical doorway" story, in which a gate between the normal world and a magical one is discovered. This formula would, of course, be used to great success years later by J.K. Rowling.
Lewis' young protagonists travel to a realm called Narnia, where they encounter all manner of anthropomorphized animals, fantastical people and a godly lion. This book leads to an entire series of Narnia novels. Like nearly every book on this list that isn't a dictionary, it was also made into a film.