Scholars can debate endlessly over which book is the "best" of all time, but finding out which ones have enjoyed the most commercial success should be a more concrete endeavor. We can just look at the sales numbers and rank them for a current list. Unfortunately, this turns out to be quite a challenge itself.
There is no central repository or current list for total sales numbers for books. Publishers may not share sales numbers for individual books, or their reported numbers may be inaccurate. It gets even more complicated when you consider that popular books often are released in multiple editions, sometimes by entirely different publishers. Older works that are in the public domain may be published simultaneously by several different companies. The fact is, any worldwide sales figures for a book more than a few years old are probably an educated guess at best.
These issues are compounded when you consider religious texts like the Bible, the Quran or the Book of Mormon. Not only are sales numbers impossible to verify, but these types of books are often given out to followers or handed out in public places. Comparing their sales numbers to the works of Charles Dickens or J.K. Rowling is like comparing apples to oranges. Therefore, these books, along with "Quotations from Mao Zedong," for example, have been excluded from this list.
With these caveats in mind, here is a current list of the 22 best-selling books of all time, as far as we can tell.