The Alchemist – 65 million
"The Alchemist" is something of an oddity on this list, and may not belong here at all. Written in Portuguese by Paulo Coelho in 1986, and then translated into English in 1993, it is touted as an allegorical tale (basically a retelling of an old folk tale) about a young shepherd who seeks his destiny in the desert.
Some sources indicate that it has sold 65 million copies. However, it's possible that number was inflated to drum up interest in anticipation of a potential film adaptation [source: AFP].