eBooks that you intend to serve as textbooks for a small or large class have some tricky access limitations working in the background. For example, academic publishers often limit the number of "seats", meaning that many eBooks can only be purchased if the number of concurrent users can be stipulated by the publisher. Sometimes it is limited to one person at a time, sometimes it is three people at a time or sometimes a few hundred uses in a calendar year. Not all eBooks allow unlimited use, hence this option is is not always available. If you would like to use an eBook as a textbook, the Psychology & Neuroscience librarian would be glad to tell you what the access options are given the publisher, so that you can decide what the best option is for your class.