According to Guinness World Records, the Bible is the best-selling book of all time with over five billion copies sold and distributed worldwide.

However, all books of a religious, ideological, philosophical or political nature are normally excluded from the list of the best-selling book since exact print figures and sale records for these kinds of books are missing or unreliable. These specific types of books have been produced for many centuries by many different and unrelated publishers.

Also, it is hard to list the best seller because "best selling" refers to the estimated number of copies sold of each book, rather than the number of books printed or currently owned by people. Typically, sale numbers for religious, ideological, philosophical, etc. kinds of books were just not recorded; they were completely overlooked and forgotten.

More Info: en.wikipedia.org