If a faculty member is using an online textbook for your course, you will be automatically opted in to get that textbook.  You should access it by going to Brightspace-->Resources Tab-->look for your textbook. If you need an access code, this will be emailed to you in a "Welcome to Inclusive Access" email about a week before classes start. [Note: Inclusive Access is the name of the program that lets you get online textbooks.]

If you didn't get that email, can't find the ebook of your textbook, or otherwise have problems, contact the BU university store.

You are not required to use the online textbooks.  You can opt out up to the date listed in your email.  You can do that online or by calling the BU university store.  It is then up to you to get the textbook in another way.