Chief Beatty, having been a fireman for a long time, knows that many firemen become curious about books and take them home. He also knows that Montag has passed this point a long time ago, and suspects that Montag is deeper into the obsession than most. When he visits Montag, he gives him a subtle ultimatum: bring the book in before 24 hours, and we won't burn your house. However, Montag has actually taken dozens of books, hiding them in his vents, and is terrified that he might have the last copy of many; not wanting to risk destroying a last copy, he thinks of a plan:
"There's only one thing to do," he said. "Some time before tonight when I give the book to Beatty, I've got to have a duplicate made."
(Bradbury, Fahrenheit 451, Google Books)
To this end, he seeks out Professor Faber, who convinces him that making copies of books is futile. Faber is too scared to act, but his knowledge and his influence enables Montag to take the next steps in his own rebellion. The plan might have worked; Faber suggests a person, but then retreats into his fear, and there is not enough time before the night is over to make the duplicate anyway. In the end, Montag turns the book over, but makes his escape and joins the renegades who memorize books.