[sgath92]

> (How do you return a borrowed ebook I wonder?)

That is actually a feature built into some ebook formats & proprietary software packages. Adobe's bloatware supports this feature, and it is actively utilized in academia, libraries, and archive.org.

Basically how it works is the software sets a limit on how many people can use the file at once. When more people want to use the file than that limit, they go onto a waiting list. In the case of libraries & archive.org, your access times out after a predefined amount of time (say 2 weeks), after which the app no longer allows you to use the file and you must get back in line to use it again. However, if you finish with it earlier, you can "return it" by volunteering to loose access sooner.

With individually "owned" copies it works basically the same way, only with only "one copy" in circulation between you and your friend who is borrowing it (while they're using it, you can't access it yourself), and access for the owner does not time-out.