What is the second best selling book of all time (after the Bible)?

The Bible, by near unanimous agreement, is #1 at over 5 billion books.

At #2 by most accounts is Quotations from Chairman Mao Tse Tung, at "billions" of copies.

To sum up why:

The book's phenomenal popularity may be because it was essentially an unofficial requirement for every Chinese citizen to own, to read, and to carry it at all times during the latter half of Mao's rule, especially during the Cultural Revolution.

https://en.wikipedia.org/wiki/Quotations_from_Chairman_Mao_Tse-tung

The Qur'an is up there in total printings too:

The Qur'an is also widely reported to be one of the most printed and distributed books worldwide, with billions of copies believed to be in existence.

https://en.wikipedia.org/wiki/List_of_best-selling_books#More_than_100_million_copies

Non-Religious Texts

According to Wiki, the top two non-religious/philosophical texts are:

1. Don Quixote - 500 million copies
2. Tale of Two Cities - 200 million copies

https://en.wikipedia.org/wiki/List_of_best-selling_books#More_than_100_million_copies

Guiness Book of Records keeps popping up as a frequent answer, but according to Wiki their sales are down at 115 million copies, below a number of other books. I guess just because you say you're #2 doesn't mean you necessarily are ;)

[ This is a FAQ and so this top thread is being used to keep up to date info on this topic ]

"Quotations from Chairman Mao Tse-Tung", a.k.a. "The Little Red Book". From 1966-1971, every Chinese adult was forced to own a copy.

I thought it was the Guinness Book of Records.

Well "The Guinness Book Of Records" is the best-selling published book. I don't know which book has been "published the most".

There seems to be more than one claim for second place. Here is one:

"I have been amazed to discover how many people believe that John Bunyan only wrote one book, that being 'The Pilgrim's Progress.' It is true that 'The Pilgrim's Progress' has been the most published book in the world, second only to the Bible, however, John Bunyan wrote much more.
http://www.chapellibrary.org/literature/bunyan/

List of best-selling books
1 The Bible - - ~ 6,000,000,000
2 Quotations From Chairman Mao Tse-Tung Mao Tse-tung 1966 ~1,000,000,000
3 American Spelling Book Noah Webster 1783 -
4 Guinness Book of World Records - 1955 -
5 The McGuffey Readers William Holmes McGuffey 1853 ~ 122,000,000
6 A Message to Garcia Elbert Hubbard 1899 -
7 The Common Sense Book of
Baby and Child Care Dr. Benjamin Spock 1946 -
8 World Almanac - 1868 -
9 The Valley of the Dolls Jacqueline Susann 1966 -
10 In His Steps Rev. Charles Monroe Sheldon 1896

https://en.wikipedia.org/wiki/List_of_best-selling_books

It is Quotations From Chairman Mao Tse-Tung - aka The Little Red Book at 900 million.

Top 10: Best-Selling Books Of All Time
Number 2: Quotations From Chairman Mao Tse-Tung - 900 million
Author: Mao Tse-Tung
Most people are more familiar with the book under its nickname: The Little Red Book . First published in 1966, it sheds light on the Chinese dictator's views on life, economics, and politics. Fairly poetic in style, Mao's fervent beliefs of the Communist ideology shine through.

askmen.com

Quotations from Chairman Mao

Quotations from Mao Zedong; collected by the PLA Daily of the People's Liberation Army and signed by Lin Biao, Chinese, 1966, 900 million.

https://en.wikipedia.org/wiki/List_of_best-selling_books

Would just like to point out that the Guiness Book of Records is not one book, but as series of them. Maybe all the issues combined add up to being second best seller, but it isn't as a single book.

Quotations from Chairman Mao (The Little Red Book)

http://vendyxiao.com/top-50-best-selling-books-of-all-the-time/

The Bible, ~5 to 6 billion
Quotations from Chairman Mao Zedong 900 million
Qur'an, 800 million
Xinhua Zidian, 400 million
https://www.mentalfloss.com/article/16406/all-time-best-selling-books

If you want non-fiction(though the quotations of Mao and the Chinese Dictionary(Xinhua Zidian) would be considered non fiction)
1. A Tale of Two Cities
2. Lord of the Rings
3. The Hobbit
4. Dream of the Red Chamber
5. Then There Were None
http://www.ranker.com/list/best-selling-books-of-all-time/jeff419

If you are referring to a non-religious book:
It is common knowledge that Don Quixote has sold over 500 million copies, and in fact, that number is conservative. It is often regarded as the second best selling book behind only the Bible, and at least conceded as the greatest selling work of fiction of all time, without out a close second,
https://en.wikipedia.org/wiki/Talk%3AList_of_best-selling_books