Digital Textbooks are the hot new thing right now. 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 10 sites that can provide digital textbooks.
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 NookColor, or the Nook Touch. B&N even have a specialized PC app for digital textbooks called NookStudy.
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.
Kno (sale) - This is a company that just sprang onto the digital textbook scene this summer. You can read Kno textbooks inside their iPad app, in most web browsers, and on FaceBook.
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.