Textbooks fall in the category of Tertiary literature, and are books written specifically for the purpose of teaching students. They contain review sections and exercises.
E-books are simply books that you can access in an electronic format. Not all books are available this way. E-books from the library are not the same as popular books you'd buy for your personal e-reader device; they often can't be downloaded in full, your period of use may be limited, and there are usually restrictions to printing pages.
Two things that are important to remember: