Glenn Wiebe, I learned about another good offering from the Library of Congress. The LOC's Student Discovery Sets are iBooks arranged around six themes. Those themes are The Constitution, The Dust Bowl, Immigration, Symbols of the United States, The Harlem Renaissance, and Understanding the Cosmos. Each theme is contained within its own iBook.
The LOC's Student Discovery iBooks incorporate tools for zooming-in on elements of primary sources and drawing on documents to highlight aspects of them. Teaching guides are available for each of the iBooks in the Student Discovery Sets.
Applications for Education
Learning from primary source documents can be challenging for many students. The drawing and marking tools available in the LOC's Student Discovery iBooks could help students identify tricky aspects of documents and write questions to ask you during class.