Without a shred of Doubt, its the Bible! Bibles have been sold, given away and smuggled into many countries. read more
More objectively, according to Guinness World Records, the Bible is the best-selling book of all time with over 5 billion copies sold and distributed as of this year. I don't know if that number includes Gideons Bibles. read more
The Bible and the Koran have both gone global. In 1900, 80% of the world's Christians lived in Europe and the United States. Today 60% live in the developing world. More Presbyterians go to church in Ghana than in Scotland. In 1900 Islam was concentrated in the Arab world and South-East Asia. read more