Formidable footprints found! Something enormous is out there. It?s too big to be human. It must be a... High-interest topics, real stories, engaging design and astonishing photos are the building blocks of the XBooks, a new series of books designed to engage and motivate reluctant and enthusiastic readers alike. With topics based in science, history, and social studies, these action-packed books will help students unlock the power and pleasure of reading...and always ask for more!