* Lack of comprehensive data: There is no central database that tracks the sales of every book ever published.
* Self-published books: Many books are self-published, and sales figures for these are often not tracked or reported.
* Out-of-print books: Sales figures for books that are no longer in print are difficult to track.
* Rarity: Some books may have very limited print runs or be extremely rare, making it difficult to determine how many copies were actually sold.
However, there are some books that are widely considered to be among the least-selling:
* "The Life and Adventures of Martin Chuzzlewit" by Charles Dickens: This book was a commercial flop when it was first published, and it has remained relatively unpopular ever since.
* "The Decline and Fall of the Roman Empire" by Edward Gibbon: This massive historical work was a critical success, but it has never been a bestseller.
* "The Complete Works of William Shakespeare" by William Shakespeare: While Shakespeare's plays are extremely popular, the complete works have never been a major seller.
It's important to remember that the popularity of a book is subjective and can be influenced by many factors, such as genre, author, and time period. Ultimately, the least-selling book ever is likely to remain a mystery.