Let's start with an open source textbook, say, economics. I know of a couple that have been approved by the state of California, which means they meet certain content requirements. Or you could find a different one or even make your own.
Next, you'll need videos. Why not try to use the lecture videos produced by the Khan Academy? They're licensed under CC-BY-NC-SA, which means you can do almost anything you want except sell them. You can legitimately cut clips out of them and integrate the clips into your textvook.
Take the CC licensed textbook, add the CC licensed clips, and you have a textvook. Of course, at this point you need to consider whether you want to go all the way and make an app, or if you want to leave the content as a website.
I lean towards an app. This creates the mind set that will point you towards having a PDF alternative. I don't want a PDF; I just want it available in case someone wants to print and bind it as a paper textbook. Yes, I know the videos won't work, but some users will still want the paper copy anyway.
Is anyone interested in trying this out?
image by ario