Our service allows for expiring transactions, so library implementations are possible. You can set a downloaded eBook to expire within a given period of time so that it is unusable after that period and is automatically 'returned'. You could also set a limit to how many copies of an eBook can be accessed at any given time.
There are two ways to set an expiration date on eBooks.
The transaction endpoint of our API deals with the expiry of the eBook itself after it's been downloaded by a user. So you have the eBooks become inaccessible by the user after a certain period of time.
You can also set expiration through the Books page on your account dashboard when adding or editing an eBook, using the "Default DRM Settings" section on your dashboard.