"Where is the book that you are reading?"
Translation:Hol van a könyv, amit olvasol?
Logically, you would think so, but the pronoun amit is not a definite object. When amit is the direct object of a verb, the verb should be indefinite. (I think the logic behind this is that amit is derived from mit, which - sensibly - is treated as an indefinite object: Mit olvasol? And so amit kind of "inherits" that behavior.)
Because in the clause where it appears, it is the direct object of the verb olvasol.