If you purchased a textbook with your course, there is no need to wait until you receive it to begin the course. Everything you need is available online. You can start your course now and download anything you need until your book comes. Textbooks should arrive within 7-10 days of enrolling.
The textbook is not required but it is a great tool to have while taking the course so you don’t have to print everything out and it is always great reference to have after you finish the course.