Reference books can be found online and in print. Most of the print reference books are on the main floor of Olin, to the right when you enter. They can not be checked out, so they are almost always available.
Reference sources are a good place to get an overview of a topic in a few paragraphs or a few pages. Looking up a topic in a reference book is an excellent way to familiarize yourself with the key concepts, terminology, and people associated with the topic. This background is important both for understanding a topic and for searching effectively in online databases for articles and books. In addition, many reference works provide carefully selected bibliographies which might lead you to further reading.