Some of our ebook systems have limited number of concurrent users (possibly just one). When you view an ebook and the limit is reached, you sometimes get a message like the above. It will eventually return to usable but the time varies based on a number of factors.
If you ever encounter this message and you did not just log off the e-book, then someone else is looking at it and you'll have to wait for them to finish. Again, just try back in an hour or so.
