"What are your favorite books?"
Translation:Quels sont tes livres préférés ?
From what I have read they both mean favorite. However, favoris is less common and more "old fashioned" sounding. Of course, that could be something one is going for, depending on the situation. I found this: https://answers.yahoo.com/question/index?qid=20071011230457AABYCYI and this: https://hinative.com/fr-FR/questions/280 I'm still learning as well, but hope that helps. :)