If you think Harry Potter is in the gold medal position of such a list you are wrong, the same as our Helgi team was.
Answering the question which book is the best selling one should be much easier then to find the best book of all time.
Unfortunately, trying to measure a book’s commercial success turns out to be quite a challenge in itself. There is no central repository for total sales numbers for books. Publishers may not share sales numbers for individual books, or their reported numbers may be inaccurate. It gets even more complicated when you consider that many books are put out in multiple editions over the years, often by entirely different publishers. The fact is, any worldwide sales figures for a book more than a few years old are educated guesses at best.
Since religious and political books such as The Holy Bible, Quran, BaghavadGita ,“Quotations from Chairman Mao Tse-tung” are often given away for free, they have not been included on this list(but for reference, according to the Guinness Book of World Records, the Bible is the best-selling book in history with over five billion copies sold).
Various independent sources gathered in wikipedia state that the best-seller of all time is the novel ‘A Tale of Two Cities’ by Charles Dickens. Originally published in 1859, 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 but 200 million is a good ballpark figure