The CK-12 Foundation has e-textbooks for iPads called Flexbooks. They are Creative Commons Licensed making them free for use by educational bodies as long as you attribute them.
At OER Commons find Free-to-Use Teaching and Learning Content from around the World. Organize K-12 Lessons, College courses and more. Search by subject area, grade level and material type.
Priced ebooks for the iPad
The list of titles available at KeyBookshop.com covers a wonderful array of topics including core subjects such as reading, writing, math, science, literature social studies and language as well as more specialized topics such as computers, technology, arts, crafts, and many others. Teacher resources including activity guides, reproducible worksheets, lesson planning guides, and holiday ideas are also available. they are available in a pdf format which could be read by apps like Goodreader.