The UC Davis library has not only a fantastic collection of books, but also a fantastic collection of searchable databases (some of which you have to be at the UCD library to search). If you're looking for books, click on the "Harvest Library Catalog" link in the upper left hand corner of their home page.
The Yolo County Library operates branch libraries throughout the county, including one here in Davis. They have many books, and offer many services and programs as well. You can also get books that live in non-Davis branch libraries through inter-library loan.
Davis Senior High School Library
The DHS library is also a wonderful place to find books and articles that might not be available at Harper.