It's a good idea to browse through entries on your topic in more than one encyclopedia. Some encyclopedias have in-depth articles, while others have just short entries. Be sure to check the bibliographies at the end of essays; they'll lead you to especially key sources.
The Nanzan Guide to Japanese Religions, listed below, is an especially good place to start, especially if you aren't sure what you want to research.
Additional Recommended Subject Encyclopedias:
Not finding the background sources that you need? Try searching one or more of these large online collections of subject encyclopedias, handbooks, and dictionaries.