When you think of digital books, I'm sure Amazon's Kindle and Apple's iBooks come to mind.
Both are good sources for digital textbooks.
And, as you already know, the services are compatible on a wide range of devices.
Amazon will let you borrow a book for a desired length of time.
Where as you have to purchase a digital copy outright from Apple.
Another good option is ecampus.com.
The site claims to carry over 100000 different titles that are available for purchase or rent.
The service let's you share information with other users and access the content through apps on your IOS or Android devices.
Inkling is another web site that synchs with your mobile device and offers hundreds of text books, especially in the business and medical field.
Readers will enjoy the easy to navigate site.
And of course be sure to check with your teacher professor to make sure he is okay with digital copies, and triple check to make sure you're purchasing the correct edition.
In San Francisco I'm [UNKNOWN] at C.net.com for CBS news.