What does Montag take out from behind the ventilator grille?

Montag has hidden books behind the ventilator grille.

Then he reached up and pulled back the grille of the air?conditioning system and reached far back inside to the right and moved still another sliding sheet of metal and took out a book. Without looking at it he dropped it to the floor. He put his hand back up and took out two books and moved his hand down and dropped the two books to the floor. He kept moving his hand and dropping books, small ones, fairly large ones, yellow, red, green ones. When he was done he looked down upon some twenty books lying at his wife's feet.