Textbooks are 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 merely books that you can access in an electronic format. Not all books are available this way. It just depends on the individual title. E-books from the library are not the same as books you'd buy for your Kindle or iPad; they can't be downloaded in full and there are restrictions to access and printing.
Two things that are important to remember: