This article from I Education Apps Review lists out a series of apps to use for creating custom digital content. Not all are free, but the list presents some interesting options.
The Virginia Department of Education helped to create this list of apps for students. You can download them from the Apple App Store. Some of them include digital texts from Pearson publishers.
Blog from Dr. Wesley Fryer - "digital learning consultant, author, digital storyteller, educator and change agent." Interesting suggestions for iPads and the arts.
This is a great blog from a group of educators in Texas. They have written a few good articles about iPad deployment as well as mobile technologies in the classroom.