Best-selling is the adjective that sits in front of a noun: a best-selling novel, a best-selling author, a best-selling album. It means the thing sells in very large numbers, the same idea as best-seller. The hyphen is the usual spelling. Same quiet trap as the noun: it talks about sales, not about whether the writing or the product is truly “the best.” Teachers and reviews sometimes blur the two, as if a best-selling thriller must be well written — though actually the charts only count copies sold. Use it before a noun; we do not usually say “this book is best-selling” on its own in careful English (we say it is a best-seller).