Once your book sells very well, what do you call it: best seller, bestseller, or best-seller?
As is the case with so many things in the English language, multiple versions are acceptable.
Look, for example, at the names for the following lists:
- The New York Times Best Sellers
- Wall Street Journal Best Sellers
- Barnes & Noble Bestsellers
- Amazon Best Sellers
So, there is some disagreement there.
What does the dictionary say?
There are multiple dictionaries with multiple versions:
So, it would seem that either “best seller” or “bestseller” are acceptable.
“Best-seller” is only necessary as a modifier as in “best-seller list,” but even that is uncommon.
Personally, I like “best seller” (two words) and “best-selling” book or author. It fits with the most popular lists available.
(Side note: Be careful about calling your book a “best seller.” As more and more authors and publishers use the term, it has lost a lot of its meaning. Plus, its relatively easy to hit one of the many, many Amazon Best Seller categories.)