For the natural world you might like
http://eowilsonfoundation.org/e-o-wilson-s-life-on-earth/ a free digital textbook available from iBooks. It has amazing animations illustrating things like how DNA works
"To create Life on Earth, the E.O. Wilson Biodiversity Foundation brought together a team consisting of educators, writers, multimedia artists, 3D animators trained in science and cinema, and textbook professionals, led by naturalist Edward O. Wilson. The editorial team, headed by Morgan Ryan, worked in full partnership with the Boston-based scientific graphics company Digizyme, Inc, headed by Gaël McGill, PhD, with the goal of creating a cultural landmark—a portal that will introduce students to the grandest story there is, the story of life on Earth, from molecules to ecosystems, from the origin of life to the modern awareness that we control the environment we live in."