The Little Prince – 80 million (or perhaps 200 million)
"The Little Prince," by Antoine de Saint-Exupéry, is a strange yet charming children's book about a prince who lives on a tiny asteroid. Published in 1943, it became extremely popular in France and then won over readers around the world. There are monuments to it worldwide, and astronomers even named an actual asteroid for the one the prince lives on in the story.
The sales number is uncertain. Some sources list it at a startling 200 million, while other list it at 80 million.