A. s.a. p! if someone asks you what the genre of a nonfiction book is, do you answer by saying it is nonfiction or do you go into more detail and say it's a biography? or do you mention both?

It truly depends on the book. It cannot be a biography if it's about natural science, but it is still nonfiction. However, if you are referring to a book about someone's life (written by someone else) then it is safe to say it is a biography. In my opinion, you can simply say it is a biography, obviously the person you're talking to will know it is nonfiction already.

Well, it all depends. If you are saying this to someone professionally, then its safe to be as specific as possible. I would incorporate both nonfiction and biography if I were you. Hope I helped!