Subject encyclopedias are much more than a way of picking up quick facts. Typically put together by scholars and other experts in the field, they can provide an overview of a field or fill in topics for unfamiliar areas of research. The articles usually also include a list of important books and articles. Not a substitute for research but a quick way of getting you started.
A few of the encyclopedias with broadest coverage are at the top of the list, but all are worth browsing. Many of these encyclopedias we also have in print.