Cassandra Clare said this quote

One must always be careful of books,' said Tessa, 'and what is inside them, for words have the power to change us.' 'I'm not sure a book has ever changed me,' Will finished. 'Well, there is one volume that promises to teach one how to turn into an entire flock of sheep-' 'Only the very weak-minded refuse to be influenced by literature and poetry,' said Tessa, determined not to let him run wildly off with the course. 'Of course, why one would want to be and fire flock of sheep is another matter entirely,' Will finished.