Answer: the mechanical hound
Step-by-step explanation:
When Montag stole some books, he turned to Faber to help him understand the books since Faber's past profession was an English professor.
Later, Faber requested moment from Montag so that his friend will help them make some copies which will be planted in the home of the firefighters.
When Montag was reading the books that he took from the ceiling vents, the mechanical hound was outside their door.