From the benefits page:
To obtain your 35% discount on books from Course Technology, just follow the link to Course for the book that you want from our books section. When checking out, enter the code GDplus to get a 15% discount on top of the 20% discount offered for ordering online.
It came to my attention that, 0.85 * 0.80 = 0.68
So you know, you get 32% discount, not 35%.