Just bought the medium-sized Ortlieb Sling-it today for that purpose (and holding bigger books). http://ortlieb.de/_prod.php?lang=en&produkt=slingit#
It looks a lot better in reality btw., esp. in medium.
Ortlieb are great products, probably the best workmanship around. All made in germany, environmentally friendly and they are extremely thought-out (strap to keep the bag to stay on your back while cycling, different surfaces, etc.