Unlike a traditional college course, you do not need to purchase or download the entire text. All of the assigned course readings are embedded in the course units.
On the page for an objective within a course unit, you will often see links to readings from textbooks, next to an icon of a book. If you click on the blue title, it takes you to a page with the assigned excerpt.
Sometimes, you will see a link near the bottom of a page for "Go to Reading" or the title of an article. This will take you to the beginning of the assigned pages or the full text on-screen, and you can use right and left arrows to navigate through the text.