I just learned that I have to use "ce mois-ci" and "cette heure-ci". Why isn't this "ce livre-ci"? Is "-ci" only needed when talking about time?
It is not false, but a bit of a heavy formulation, that we would probably avoid unless you want to put emphasis.
Think of it this way:
"Ce livre" can mean both "this book" or "that book" and is up to the récipients interpretation to déterminé if you meant this or that
The rule you just described would be if you want to exclusively refer to it as "this book" or "that book"
Ce livre - this/that book Ce-livre-ci - this book Ce-livre-lá - that book
Duolingo prefers the alternative that's most similar to the French word. Be grateful and take advantage of the French influence on the English language. Auteur and author are identical words, whereas writer and author are not. A clerk, for instance, is a writer, but not an author.
This is a situation where you will find that invoking natural English expressions to correctly translate the meaning of the French will serve you well. We generally say "the author of a book", not "the writer of a book". Although "writer" is not literally wrong, "author" is preferred. "Writer" = écrivain, i.e., a professional writer. "Auteur" may be "writer" but it does not refer to the profession of "writer".
- Es-tu l'auteur de ce livre ? = Are you the author of this book?
- l'auteur de cette lettre = the writer of this letter
- Elle est écrivaine = She is a writer