Reference books -- either in print or online -- are great for finding background information on a topic. Find out biographical information on an author; key dates, socio/political history of a time period, important events, names of important scholars in the field, and a helpful bibliography for additional reading.
Here are a couple of options for finding reference material online:
What is ILLiad?
Keep track of your citations as you find them. Use RefWorks to organize your research!
RefWorks is an online citation management tool that allows you to create a personal database of references by importing them from research databases. What's the best part? You can change citation styles or generate bibliographies in seconds.