Wht did Montag do in the old lady's attic?

He took a book:

Montag had done nothing. His hand had done it all, his hand, with a brain of its own, with a conscience and a curiosity in each trembling finger, had turned thief. Now it plunged the book back under his arm, pressed it tight to sweating armpit, rushed out empty, with a magician's flourish! Look here! Innocent! Look!