Montag, a fireman charged with the responsibility of burning books, was convicted by Clarisse’s probing to question the purpose of his job. In his quest to learn more about what books contain and establish the genesis of book burning, he stole several books the first of which he hid under his pillow as he and Mildred slept on separate beds. Afterwards, he hid the rest of the collection in the ventilator grill. One day Mildred’s friends come over to Montag’s house to chat and watch their usual shows and Montag, angered by their superficiality, switches off the walls and reads them poetry by Mathew Arnold. This angers and saddens them and they are forced to leave. After this Montag realizes he has made a mistake and goes for his book collection in the ventilator grill and "He carried the books into the backyard and hid them in the bushes near the alley fence."
Montag realized he had made a mistake in reading the poetry to the ladies. Faber emphasized the point when he said into the earpiece,
"Fool, Montag, fool,fool. oh God you silly fool..." (pg 101)
Montag finds his books hidden behind the refrigerator. Mildred, in fear of losing her home, his job, and her life as she knows it, hid them and was burning them one by one. Montag realizes that he has to move them if he wants to save them. He moves them into the backyard and hides them in the bushes near the alley fence. He just plans on leaving them there for the one night in case she decides to do any more burning. He has to go to work. Remember, all of their fires were at night.
Ironically, that very night they stopped and burned his house, and it was his wife and her friends who reported him to the fire dept. He never gets to retrieve the books.