This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
The current estimates that there are over 40 million Terry Pratchett
books published worldwide at the moment. This will probably increase
at a rate of a million per year. (A LOT of books)
If each Pratchett book was placed on top of another Prachett book -
There would be over 40 million of them! (you didn't see that joke
coming did you!)