What is a Reference Book?
Reference materials are sources that provide a broad overview on a topic. They are typically "scholarly" and written by experts in a field, HOWEVER they aren't always considered "peer reviewed" are should not usually be cited directly in a paper. They are excellent starting points and will direct you toward useful peer-reviewed materials. You typically won't want to use a reference work as a cited source in your works, however they are excellent if you need a quick overview on a topic, or if you need to quickly identify seminal works (which you can cite!) on your topic.
A Few Philosophy Reference Materials of Particular Note: