Some publishers, such as NetLibrary, only allow one user at a time to look at an e-book. Other publishers, such as Taylor and Francis, may also limit use to a small number of users (in this case 5) at any one time. You will be able to access the item when the other user(s) have logged out.
If you are using EBSCO, you can create an account and then place a hold on a title and you will be alerted when the e-book becomes available.
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