What is hidden in Montag's ventilator grill of his house?
Montag has books hidden in his ventilator grill at his house.
Although he has not read any books before meeting Clarisse, his brief encounter with her causes him to be reminded of these books when he returns home:
He stood looking up at the ventilator grill in the hall and suddenly remembered that something lay hidden behind the grill, something that seemed to peer down at him now.
Montag must have had some spark of curiosity about the contents of the forbidden objects that he burns or he would not have stolen them. Now, after talking with the girl named Clarisse, Montag begins to wonder what it is that the books contain. When the books seem to "peer down at him," he "move[s] his eyes quickly away" as though he wishes not to think of them, or, perhaps, out of fear that he does desire to examine them. Clarisse has awakened something in Montag, something that he wants to experience, but, at the same time, he fears doing so because it is forbidden.