UK book discovery and selection site Lovereading has developed this striking – though pretty conjectural – infographic of the most popular books of all time, based on number of copies sold, translations made, and editions printed. For some titles, especially the Quran and the Bible, these figures are certainly going to be somewhat off target, even more so since the data is just over a year old, but the general picture is clear. I’d really hope against hope that the data on the Quotations from Chairman Mao or The Da Vinci Code is off though.
The bible is going to be even more off than you thought, because that’s the just the King James version – there’s a lot more versions out there, some of them selling very well.
I think the Quotations of Chairman Mao is likely to be accurate – note that the vast majority of those will have been sold in China. There used to be quite a lot of pressure to be able to quote from it, have access to a copy, etc.