eBooks that you intend to serve as textbooks for a small or large class have some tricky limitations on use 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 hundreds of uses in a calendar year. Expecting the library to buy one book that allows unlimited use is not always an option. If you would like to use an eBook as a textbook, the math 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.