I picked up this book to go along with our nature study on oaks and it is perfect for that.
"Oak Tree" by Gordon Morrison is beautifully illustrated and full of information. It covers facts about the tree, but also about its inhabitants and the changes that happen through the seasons and its life cycle.
"A woolly bear caterpillar wriggles across a colorful oak leaf. Oak bullet galls form on the twigs. And the old tree moves less and less sap as each passing day grows shorter and colder."