This year, Houghton Mifflin Hourcourt is piloting an Algebra I iPad textbook in select California public school classrooms.
The e-book, called the HMH Fuse: Algebra I, is more than just a pdf of the paper textbook. The e-book contains self-tests where students get feedback on how well they understand a section, videos where a nerdy-looking teacher figure explains concepts, and demonstrations where students can see how changing the m in y=mx + b affects the graph of the line. Here is a video that shows how the textbook works.
I will be interested to see the outcomes of this pilot project. How will the student learning with the e-books compare to the student learning with the regular textbooks?
Regardless of the outcome of this particular experiment, I do believe that this is the way that all textbooks will be going very soon.