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".