Re: Why not something reasonable like a Kindle?
Good digital textbooks are more than just a pdf (though I have no idea if these were or not.) Having a digital "textbook" gives the writer a lot of opportunity to include interactive materials. You can allow the student to interact with the examples more directly, instead of just the standard way. A simple example would be a econmics text that included graphing software to allow students to change variables in the function they're studying to see how the results change. Students could also play games where they use the knowledge that they're learning. A good digital "textbook" will encourage the student to interact with and explore the concepts being presented rather than just providing a static view of the content.
The downside being that this type of "textbook" takes a lot more work to produce and thus costs more. They should probably be produced by small groups with individual people focusing on different parts (graphs, charts, models, 3D graphics, writing,) but AFAIK, they're still each written by one guy who is forced to learn how to create the interactive parts too.