You will need textbooks at least a few days in advance of the first class session -- many classes have reading assignments due the first day. You'll receive access to course syllabi on Thursday of orientation week, so you'll want to have your books so you can start any reading assignments over the weekend.
You are not required to order your materials before arriving on campus, but you may want to do so to obtain a competitive price. You can buy on campus or through other vendors of your choice, including Amazon.com or other online retailers. All materials are available, at a minimum, either through Iowa Book & Supply or the University Bookstore unless otherwise specified, though you may find better prices through online vendors.