The UCSD Bookstore is pretty decent as far as campus book stores go. They carry most of the text books you may need for class, plus have a decent selection of other books, some art and office supplies, a small computer/electronic store, and of course a lot of UCSD gear (t-shirts, sweatshirts, postcards, etc).
If you're looking for books, you may also want to check out Groundwork Books, the General Store Co-Op, or various online or area book sellers. The General Store Co-Op also carries UCSD gear.
The Sunshine Store is attached to the bookstore.