There are several different species of textbook. The first and most common is the traditional print book. But many publishers now offer students the option of buying the "traditional print book" in a digital format. These etextbooks are often specifically designed to be read on a tablet or personal reading device like a Kindle. Though they reproduce the content of the traditional print book by and large, etextbooks might include some multimedia content. These must, of course, be purchased from a publisher and, for that reason, can be rather expensive.
Less expensive options include open-source books and free textbooks, many available on sites like archive.org or scribd. It is also the case, however, that an increasing number of authors are making their own online textbooks freely available after the publisher's copyright has expired and reverted back to the author(s). This varies on a case-by-case basis, so instructors are well-served to stay abreast of the latest trends in their field. Frequently a popular blog or website for the field will announce when a new book has been made freely available by its author(s). This can, of course, be an attractive option for instructors, since they garner all the advantages of a free textbook without any of a traditional textbook's frequently prohibitive costs.
Instructors are always well-served to look at the syllabi of colleagues, particularly at other institutions, as the syllabi might well offer ideas and opportunities for textbooks -- both traditional and open-source -- that might be otherwise overlooked.
Kindle Paperwhite
The Kindle Paperwhite is one device upon which students may access an etextbook.