Over a few years, Montag has collected over twenty books; he did so unconsciously, not understanding what compelled him to save those books from the fire, but knowing that something about them was more important than society allowed. When he realizes the scope and potential of owning all those books, he and his wife Mildred are scared that they will be identified and killed. Montag makes a plan; he knows that Beatty saw him steal a book, or at least suspects it, but thinks that he can turn in one book and get off the hook, as Beatty explains earlier.
"It might be the last copy in this part of the world."
"You've got to hand it back tonight, don't you know? Captain Beatty knows you've got it, doesn't he?"
"I don't think he knows which book I stole. But how do I choose a substitute? Do I turn in Mr. Jefferson? Mr. Thoreau? Which is least valuable?"
(Bradbury, Fahrenheit 451, Google Books)
Despite this, Montag doesn't want to destroy any of the books, because they might be the last copies in the world. Instead, he tries to get Professor Faber to contact a printer, who might copy the books and so save them; however, Beatty sees through Montag's plans, and forces him to burn his own house and books. Montag's plan was immature; he thought that simply saving the books themselves would help fight the future society, without fully understanding that he needs to change people's minds and help them understand the part of their lives that is missing.