I'm correcting a German-English translation for an exam, and the German word Schriftsteller appears in it. My sense is that this corresponds to "writer," and that there is a difference between "writer" and "author," even though the two terms can refer to the same person.

I would articulate this difference as follows: a writer is someone who writes books (as a profession?), but an author is someone who has written one particular book, or a set of books. So one would say "Jane Smith is a writer," but one would say "Jane Smith is the author of Title of Her Book," or "Jane Smith is the author of four books."

Comments welcome on this distinction, of course!