When you purchase e-books from the DU Bookstore, you should get an email in your DU Panther Mail with instructions on how to access them. 

If you do not receive an e-mail, you will need to verify that your e-mail is registered with the bookstore.  Instructions to do that are in the video on the Course Materials page under the header Tips for accessing materials in Blackboard.  

This page also includes instructions for accessing your e-book through VitalSource or Cengage.

If that does not resolve the issue, you will need to contact the bookstore.