There have been quite a few projects to try to solve link rot but they've met with moderate success at best. The comic references archive.org, which is the most successful to date.
I think a decentralized web like what Maidsafe promises would help in making sure that at least un-maintained websites don't go down.
Another solution would be allowing conversion of resources to PDF from proprietary formats like what Inkling uses. That would ease my mind as well because if Inkling goes under for some reason, then I will have lost all of "my" medical texts that I bought.
The rich media (3D models, dynamically-generated quizzes, etc.) would be lost but I'd take that loss over losing the whole textbook!