The likely cause is due to the limited number of seats that we have to Safari Books Online. We get access to Safari via the VIVA consortium, which consists of most of Virginia's 4-year colleges, as well as VCCS. However, all those schools together share a total of 52 simultaneous users, and if we hit that limit, users 53 onwards are unable to access the books. Furthermore, after one of the users leaves Safari, they keep that seat reserved for 30 minutes or so, so that they can resume reading in case they accidentally closed the window or had some other technical issue. Sadly, their error messages don't make that reason clear to our users. Usually we run into this problem if one of the books is assigned for a class, resulting in higher than normal usage.