Take a look at Mobipocket eBooks and MS Reader eBooks... Those are eBooks a lot of people have probably lost access to.
**Raises Hand** and some that were Adobe's locked PDF format from the early '00's. The early SCE e-books I bought are now unreadable. That got me out of the e-book world for the better part of a decade.
When I came back, I made sure to figure out how to unlock the books I BOUGHT lest the format-provider decide to stop supporting my books again.