French grammar: Interrogative adjectives

Sometimes French grammar is much stricter than English grammar. A simple question like "What book do you want?" is technically incorrect, because in proper English, the question should be "Which book do you want?" In reality, the former is much more common than the latter. In French, however, one does not have this option: the French equivalent of which, quel, must be used whenever there is more than one noun that you are choosing between.
More: French adjectives | Asking questions