This Harvard Square staple has been around forever, selling absurdly overpriced textbooks to a captive audience of Harvard students. In addition to the reading for Physics 10b, you can find everything and anything for the average dorm-dweller, or even tourist, at the Coop.
There are two locations in the Square - one is a comprehensive bookstore, located prominently on Mass Ave just across from the T stop. A few streets back you'll find the other Coop (they are actually connected by an overhead walkway), where you can browse through three floors of textbooks, prints, Harvard paraphernalia, and even makeup.