For the student and scholar, reference can have two meaning. One, a source that you can refer to for answers, and also the source, or reference, that accompanies a book or article. A good encyclopedia article gives you the best of both.
For "ready" reference - quick answers or definitions of terms, try Credo Reference.
Browse here for philosophy encyclopedias.
Remember to check the bibliography at the end of an article - a basic trick to build your own bibliography for your paper. Browse the master list of print & electronic encyclopedias available to you as a student for more great sources for your other classes.