The 20th century: era of the print bestseller, before the internet and technology changed reading habits for good.

The average person read more than today, and read non-fiction - including a lot of self-help and motivation.

12 books that were massive in the 20th century:

Quotations from Chairman Mao Tse-Tung (Mao Zedong) (1964)

Also known as the Little Red Book, this book was required reading for all Chinese citizens during the Cultural Revolution.

Hundreds of millions of copies printed.

The Guinness Book of World Records (1955)

Originally published as a reference book for pub quizzes, it became a household name and has sold over 141 million copies.

Think and Grow Rich (Napoleon Hill) (1937)

Motivational self-help book that has sold over 100 million copies worldwide and has been translated into more than 30 languages.

The Common Sense Book of Baby and Child Care (Dr. Benjamin Spock) (1946)

Bestselling guide to raising children, this book has sold over 50 million copies and has been translated into more than 40 languages.

How to Win Friends and Influence People (Dale Carnegie) (1936)

Classic self-help book and guide to human nature that has sold over 30 million copies worldwide and been translated into more than 30 languages.

"The Diary of a Young Girl" by Anne Frank (1947)

Diary of a young Jewish girl who lived in hiding during the Nazi occupation of the Netherlands.

Sales of more than 30 million copies worldwide and has been translated into more than 70 languages.

"The Seven Habits of Highly Effective People" by Stephen Covey (1989)

Teaches readers how to improve their personal and professional lives by developing effective habits. It has sold more than 25 million copies worldwide.