Where and when did Montag steal his first book?
Bradbury doesn't make it clear when Montag stole his first book. We know from very early in the story that Montag has something hidden behind the vent in the ceiling and it soon becomes obvious that it's a book(s). From Beatty's explanation, later in the first part, that most firemen become curious about what they're burning and pick up a book to look through, we understand that is probably why Montag has stolen a book and hidden it. He probably wanted to see what was so bad about these books they burned and he most likely took one or more home to peek at the forbidden fruit. That's human nature to be curious about what is prohibited without clear explanation as to why it is prohibited. It is Clarisse's question about whether or not Montag is happy that gets Montag to question his life. Then, when he sees the old woman who would rather die than give up her books, he realizes that there must be something special about those books that they burn.