When you first click on a new VitalSource course textbook, you are granted temporary access for about 2 weeks- this covers the add/drop period at the beginning of the term.  To make sure you have the long-term access to the book, you need to click on the link in your course shell during the third week of the term. 

If you have downloaded the book and then it disappears, this is usually the problem.  Go to Canvas and click on the textbook link again.