I really get geeked out when we start talking about new technologies removing barriers in the energy, education, and healthcare fields (among others).  This article certainly counts.  21st century textbooks won't (can't!) just be pdf's ... we have the ability to reduce the time from authoring to consumption, we have the ability to create educational experiences, and we have the responsibility to make those experiences:

• Useful: drawing on sounds pedagogies
• Usable: facilitating the student's ability to learn, removing all possible frustration with the tool and allowing the student to focus on learning
• Desirable: Fun! Stimulating! Not boring!

Without further ado ... the article!

The 21st-century textbook - O'Reilly Radar

In addition to living, interactive, participative, adaptive, and connected, we can expect the 21st century textbook to be personalized and mashable.