Which novel is the best selling of all time — with over 500 million sold?

The novel which is the best selling novel of all time - with over 500 million copies sold - is Don Quixote.
The novel was written by Miguel de Cervantes in 1605 (and the second volume in 1615). It is about a landlord called Don Quixote, whose delusional mental disease made him think he is an errant knight on a quest to defend his lady Dulcinea's honor by having to fight all sorts of enemies, like windmills (which he thought were giants) and such. 
Although it is not the most selling book of all time (the Bible is), it is still the most selling novel, followed by A Tale of Two Cities by Charles Dickens, and The Lord of the Rings by J. R. R. Tolkien.