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.
Montag has hidden a small collection of books behind the ventilator grill in his home. Montag is a jaded firefighter who has been questioning his life and occupation after meeting Clarisse. Montag has been dissatisfied with his life for some time and has been secretly taking books while on duty. Books are illegal to own in Bradbury's dystopian society, which is why Montag is forced to hide the novels behind his ventilator grill. He believes that the ventilator is an adequate hiding spot to place the books, and he stares directly at the grill several times in part one when he begins to question his happiness. Montag believes that the answers to many of life's most difficult questions could be written in books and contemplates quitting his job. After having a tense conversation with Captain Beatty, Montag takes the hidden books out of his ventilator and begins reading them.