Many commercial e-textbooks are not available for academic libraries to purchase. Publishers choose to make their e-textbooks for sale directly to students rather than libraries. As a result, when a textbook is assigned for a class, there may be a chance that the library is unable to purchase it. The University of Alberta Library has an official statement about this issue here.
Publishers that will not sell e-textbooks to us:
Thinking of assigning an ebook for a class, but it's not available? Explore alternatives like OER, which are teaching and learning materials that are freely and openly available. More information here.