Sometimes you will see a message like this when you try to access one of the Library's e-books:
Most e-book vendors only allow a limited number of viewers for an e-book at one time. When you see a message like the one above, it means that the number of viewers has reached the limit, and you will need to wait until one of them exit the e-book before you can view.
In some cases, other Library users may have checked out and downloaded the e-book, so you may have to wait for up to one day before you can get access.
Think of it as borrowing a physical book at a library. When all the copies for a particular book are checked out, you just need to wait until one of the users return the book to the library before you can borrow it.