The world's best selling single-volume book is 'A Tale Of Two Cities' By Charles Dickens (Over 200 million books). The best-selling book series is of course 'Harry Potter' having sold more than 450 million books.
The Bible is the most popular book in the world, but many are given away for free. A Tale of Two Cities has sold over over 200 million copies. The Harry Potter book series has sold over 450 million copies.