Online vs hard copy, which texts are students more likely to read?
This recent literature review goes over some of the research on print versus screen reading.

It looks like in real-world situations there are no significant differences in grades between e-textbooks and print textbooks even though some laboratory studies show that content reading works better from print and that some students tend to prefer print.

In contrast, course reserve books are more likely to be accessed if they're e-books rather than print books. 

Practically, it may be worthwhile to give students a "tour" of the ebook to make sure they know how to use it, and/or to design quizzes or activities that require students to know/use the textbook material.