Everything you need to learn PhotoReading is in the book and some people do learn from the book alone. The course teaches it step by step with exercises that are not in the book.
If you happen to be someone who can learn with just a book then you probably would know how to PhotoRead and use it.
The audio course takes you through the process in the time it takes you to become a PhotoReader. Most people just using the book don't apply the time nor are they creative enough to develop experiential exercises. The course is developed for home study. If you just sit back and listen to the audio course then it will appear that it is just another version of the book. If you do the exercises as presented in the home study course then by the time you've finished listening you are a PhotoReader. A significant difference between just reading a book and having a way to experience the subject.
You'll find the question is often asked on the forum. Can you learn with just the book? The answer has always been Yes. The home study course just makes it easier and for many faster.
Alex