Best-selling books. Which part of speech is "best-selling" and why?

Question

https://en.wikipedia.org/wiki/List_of_best-selling_books

I was very confused and curious about why we use Active Voice instead of Passive Voice here. I mean, a book on its own can barely sell anything! People sell them.

They explained to me it's because the word "selling" is a noun in this situation.

But could someone explain it more explicitly? Why is it a noun? I have some trouble determining the part of speech of those -ING words. The situation is even worse in my case because we have "best-selling" rather than just "selling".

Answer 1

They are our [best-selling books].

"Selling" is not a noun. "Best-selling" is a compound adjective consisting of the adverb "best" as the first component and the gerund-participle verb "selling" as head.

"Best-selling" then modifies the noun "books".

Answer 2

In the noun phrase “best-selling novel” the (only) noun is novel. And “best-selling” modifies it:

”Say, Bill, how well is your novel selling?”

”It’s selling better than any other!”

”Wow! It’s the best-selling novel?”

If selling were acting as a noun (a gerund) it would be naming the process or activity that happens as things are sold, as in “Selling books can be hard work.” Instead, it’s acting as a verb (a present participle), naming what’s being done to the book: the exchange for money. Compare to a similarly constructed phrase that involves a past participle: “best-loved novel.”

And by the way, in “How well is your novel selling?” as in “best-selling novel,” the verb “selling” is in the active voice. It’s an intransitive use of the verb “to sell.” Compare to, “Milk is selling for $4.95 a gallon,” or “Champagne sells well around the holidays.”