There are two types of eBooks: single-user and multiple-user. Multiple-user titles allow an unlimited number of people to access them at the same time. Single-user titles may be used by only one person at a time. Most of the titles in the EBSCO eBook Collection are single-user. Most titles in ebrary are multiple-user. You can tell if a title is a single-user in ebrary if they have a “Release This Title” button in the QuickView window.
If a title comes up as “not available” you can put a hold on it and be notified when it has been released by the other user.