>>How do you know whether a book is a bestseller?
>
>Good point. I suppose when it appears at the front of an airport bookshop, multiple copies facing you.
>
>Maybe it's only a bestseller, judged by the hardback sales, once it comes out in paperback?


There are separate bestseller lists for hardback (we call them hardcover) and paperback. Also for fiction and non-fiction, and then again something called trade paperbacks. Those have the form factor of hardcovers but have paper covers and are priced about halfway in between.