There's an abundance of materials online, the issue is that they're not collated or aggregated in such a way that a technically uninspired teacher could effortlessly utilise them.
This is a problem that I and many others are working towards addressing.
"Textbooks" however, being books written to directly accompany a course, are probably not going to constitute the prevailing paradigm online - they are static and structured in a way that traditional printing required but modern technology finds burdensome.
Actually what I'm working towards is much like what jvincent is asking for:
"students need to be able to self-teach using online dynamic material (as opposed to static textbooks). Teachers would design courses from mashups and assist students individually in their progress"
Mine would be an interface for designing and delivering those "mashups".
I plan to release much of it on an MIT (or equivalent) license, so if you're a python/django coder you can contribute code to it. Otherwise, your feedback would be great, once I have something to show you - you can get in touch with me through the link in my profile.
We've built a document editor that might be suitable for reading (and writing) textbooks. It allows the text to refer to figures and details within those figures (e.g. "In Fig. 3, roller 22 is mounted on support 24.") It's originally built to prepare patent applications (see teampatent.com and ask for a beta account) but has wide applicability in other fields. It's collaborative like Google Docs, runs on Amazon Web Services so it can scale, and is built exclusively with open source (with a Python backend) so we have flexibility on how to deploy it. I can't reach ozanonay at her website so contact me at rocky at teampatent dot com.
This is a problem that I and many others are working towards addressing.
"Textbooks" however, being books written to directly accompany a course, are probably not going to constitute the prevailing paradigm online - they are static and structured in a way that traditional printing required but modern technology finds burdensome.