IDL has an agreement with The University of Georgia Bookstore to stock textbooks and materials used in our courses. You can order from their website at http://www.ugadl.bkstr.com or by phone at 706-542-3171.
You are certainly welcome to purchase your books anywhere you can find them. The important thing is that you get the right book(s) and that you purchase them as soon as possible.
Some IDL courses have supplemental readings referred to as "course packets" in the list of required textbooks. These supplemental readings for the course can be accessed in an electronic format from the University System of Georgia Library site for no charge. Instructions on how to access these readings will be included in your course materials upon registration. | 
