"Which book are you reading?"
Translation:Hangi kitabı okuyorsunuz?
Fist off, you would have to use "siz" in this sentence because of the verb ending. You have two places where it could go, depending on what you want to say.
"Siz hangi kitabı okuyorsunuz" would be the neutral way to ask (stressing "which book") "Hangi kitabı siz okuyorsunuz" stresses "you." (Which book are YOU reading [as opposed to me or them])