Bradbury is speaking figuratively in this statement. Montag isn't completely aware of his actions. He finds himself taking the book without thinking. In his mind, he knows that what he is doing will get him in trouble. Yet, there is the growing and insatiable desire in Montag to gain more knowledge through the books.
This scene helps support one of the main ideas that Bradbury is developing through the novel. By Montag taking the book without thinking about it, Bradbury is emphasizing that in man's core he wants to pursue knowledge and think for himself. Its an innate desire. That desire is often stifled, however, by a competing sense of obligation to society, specifically to not offend anyone at any time.
As was mentioned in the previous post, Bradbury infuses Montag's hands with a life of their own to illustrate Montag's inherent desire to seek and gain knowledge. Although Montag physically grabs the book from the woman's collection, Bradbury writes,
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.
Montag's instinctive reaction to steal a book reveals his inner conscience. Deep down Montag desires wisdom and knowledge, which is why his curiosity cannot be contained. Bradbury personifies Montag's hands to depict the inner struggle Montag faces on an everyday basis living in the dystopian society. Montag understands that he is committing a crime, but cannot help himself. Montag's hands essentially act as his conscience. Although Montag realizes that it is illegal to possess a book, he cannot help but steal one. Montag's desire to gain wisdom and knowledge are revealed when his hands autonomously take a book.