Reference works can be extremely handy to help give form or focus for your topic and to identify quickly some key words, authors, and/or titles with which to begin online searches. Many reference works are now online in collections, e.g., Gale Virtual Reference and Oxford Reference Online so you can search many at once, including those in related disciplines. Also try Reference Universe which is an online index of both electronic and print reference works so it is even more comprehensive.
Increasingly, faculty approve of Wikipedia as a source -- with caveats. It can be useful as a quick way to look up terms or simple facts. For complex research topics Wikipedia can even be useful in offering a basic orientation, and links in an article can help guide the user to deeper or more contextual knowledge. It is advisable not to rely on Wikipedia exclusively or as the most authoritative source. Subject experts will offer more authoritative information, interpretation and richer context. Compare, for example, the Wikipedia article on Abraham Lincoln with the same article in the American National Biography. Wikipedia Roll offers a visualization of the constellation of connections for a specific search topic that helps give an overview; the example in this case is based on a search of "religious studies." Eyeplorer offers an alternative type of visualization.