I’ve searched the internet long and deep for this, but I couldn’t find an answer with any authority to speak of. The Bible and the Koran are obvious contenders, but I’ve also heard people name the IKEA catalog. Or perhaps Mao’s red booklet, the Da Vinci Code, Harry Potter? I’d really like to know if anyone has a clue on this topic.
http://www.ipl.org.ar/ref/QUE/FARQ/bestsellerFARQ.html
That site lists in order, The Bible (estimated at 6 billion copies), Mao’s Little Red Book and American Spelling Book (Noah Webster) as the top three.