The best-selling book of all time is the Christian Bible.
It is impossible to know exactly how many copies have been printed in the roughly 1,500 years since its contents were standardized, but research conducted by the British and Foreign Bible Society in 2021 suggests that the total number probably lies between 5 and 7 billion copies.
https://www.guinnessworldrecords.com/world-records/best-selling-book-of-non-fiction
And it is also one of the first books Peter Gamma from www.petergamma.org
has read as a child.