Digital Textbooks are the hot new thing right now (or at least they were 4 years ago; in June 2015 the heat has cooled).
I don't care for them for any number of reasons, but it looks like a lot of students do. Here is a list of a dozen sites that can provide digital textbooks.
This list was updated in June 2015.
Amazon Kindle (rental, sale) - Amazon just got into renting digital textbooks in August 2011, but they've been selling them since November 2011. You can use Kindle textbooks on the Kindle and on the many Kindle apps.Ebooks are sold in Kindle and Kindle print replica formats. Note: don't buy a Kindle Print Replica textbook until you know that it will only work with Kindle for PC/OSX. KPR textbooks do not work with the Kindle or most apps.
Barnes & Noble Nook (buy, sell, rent) - Here's one that should need no introduction. B&N buy and sell used and new textbooks and they also sell digital textbooks. You can read the digital textbooks with any of B&N's Nook apps, the Nook Glowlight, or the Samsung Galaxy Tab Nook.
iBooks - Apple got into digital textbooks in a big way in early 2012. They only sell textbooks in their own format, but they do offer richly formatted titles with features similar to Epub3. Those textbooks can be read on the iPad. Apple also offers iBooks Author, an OSX app for making textbooks.
Cengage Brain (sale, rental) - Cengage sell and rent paper textbooks and they also rent digital textbooks. The latter are sold in a proprietary HTML5 format that should work in most web browsers as well as their iPad app. The textbooks are offered in both an online and offline mode.
Chegg (rental for both paper and digital) - This is a really popular site right now and they just started renting digital textbooks in August 2011. You can buy, sell, and rent paper textbooks too. Digital textbooks are read in a web browser and require an active internet connection.
Coursemart (rental) - This is one of the big names in digital textbooks, supposedly. They only rent textbooks, and you can read them on Windows, OSX, Android and iOS. The textbooks are offered in both an online and offline mode.
DigitalTextbooks.com - This site has been renamed Xplana, but it's still owned by the same parent company. I'm uncertain of the format sold but i do know that the digital textbooks are read with the Xplana browser app or the iOS app. It looks like they also sell used and new paper textbooks.
eCampus.com - They buy, sell, and rent paper textbooks. You can also rent textbooks from them in CourseSmart or Vitalsource formats.
Textbooks.com (sale)- You can buy, sell, and rent paper textbooks from this site and you can buy and digital textbooks. Ebooks are sold in VitalSource and Adobe DE formats.
Vitalsource (rental) - This is the other big name in digital textbooks. They only rent textbooks, and you can read them on Windows, OSX, and iOS. They also report that you can read the textbook in your browser, and that means they also support Android.
Zinio - This digital magazine distributor have been selling digital textbooks since forever, but hardly anyone knows about it. You can read Zinio textbooks in Zinio's iOS, Android, and PC apps.
Yuzu - This is B&N's new digital textbook platform. As of June 2015, it is still shit and I don't recommend using it. It sells textbooks in a unique proprietary format and has apps for Windows 8 and the iPad.