Reference books -- refer to these resources when you are looking for factual, authoritative information. If you are writing about a topic you know little about, sometimes reading an article from an encyclopedia can help you to generate keywords and get a foundation of information.
Online Reference Book Collections:
Gale Virtual Reference Library Rohrbach Library's longest shelf of reference books continues to grow as new titles and sets are added.
Oxford Reference Center A collection of over 175 reference books from the Oxford University Press.