The best-selling work of fiction isn’t Harry Potter.
As of 2011, the Harry Potter series of books is believed to have sold in the region of 450 million, but on an individual basis, none of them come close to Charles Dicken’s, A Tale of Two Cities, which has sold over 200 million copies since 1859.
The only work of fiction that comes close – and may yet surpass Dicken’s tale – is J.R.R. Tolkien’s, The Lord of the Rings. Although frequently sold as a series of three books it was originally intended by the author to be published as a single tome. Despite being published almost 100 years after A Tale of Two Cities, it has already reached 150 million sales.
Wikipedia: List of Best-Selling Works