What is the best-selling novel of all time? According to this article in the TLS, till recently, Wikipedia said it was A Tale of Two Cities. As the article states, this was added to Wikipedia, removed, add, removed… It now seems to have been removed and Wikipedia offers us Lord of the Rings, with the qualification that it is impossible to verify for many books as there no reliable sales figures. While it may been removed from the English Wikipedia, it still remains in others: Spanish, Portuguese and Italian, for example (I have not checked them all.) However, this is not going to stop others from making claims. The Guardian suggests Fifty Shades of Grey may the UK’s bestseller but maybe not. It may be The Da Vinci Code . No mention of Dickens. This list and this one go for A Tale of Two Cities, with no source given.
But what do other countries think? FNAC in France has Don Quixote as the best-selling novel of all time, followed by Harry Potter and Dickens in third place. This list favours Dickens, however, as does this one.
Dickens seem to win in Germany and here as well, beating out Le Petit Prince. The Spanish, probably copying English-language sources, favour Dickens: here and here but this Peruvian site favours Harry Potter and this Spanish site favours The Lord of the Rings. Interestingly, this site lists the 10 most read books and the top novel is Don Quixote followed by, interestingly enough, Cien Años de Soledad (One Hundred Years Of Solitude).
The Portuguese favour Don Quixote here (though putting The Count of Monte Cristo second) and here while the Brazilians are Dickens fans. The Italians also like Don Quixote: here and here, probably from the same sources as the Portuguese ones, though Dan Brown tops this list.
So what are we to conclude from this? The answer is simple. We do not have a clue what is the best selling/most read novel of all time. With the possible exception of Cien Años de Soledad (One Hundred Years Of Solitude), none of the candidates is or is likely to be on this site. The best-selling novel will be one that has been translated into many languages, pirated numerous times (and well before e-books), appeared on many school/college required reading lists and is still much read. But which one of those mentioned, we do not know.