Download Adobe Reader to your computer for free (see Resources below). The Reader program allows you to view Adobe PDF files. Adobe PDF is the industry standard format for creating and viewing eBooks because you can't edit the file and it's easily displayed onscreen for scrolling.
Find and buy the textbook that you need at DigitalTextbooks. Browse for your textbook by subject or by your school. There are not many websites that sell textbooks in eBook form in addition to hard copies of the book. You can also go to the textbook publisher's website to see whether there is an eBook version of your required textbooks.
Download the digital e-textbook to your computer and save it for later use. You may only be able to access the file by logging into the Digital Textbooks system, depending on the rules set by each publisher.