A Tale of Two Cities – 200 million
First published in 1859, this novel by Charles Dickens examines the class struggles that lead to the French Revolution, and the uncomfortable truth that sometimes the revolutionaries are worse than the establishment. "It was the best of times; it was the worst of times."
Because this book is in the public domain and has been published in numerous editions by hundreds of publishers over the last 150 years, it's impossible to ascertain the exact number of copies sold. The number 200 million is controversial -- however, considering that the book is regularly on the required reading lists of students in the U.S. and elsewhere, it's entirely plausible.