Reference materials (encyclopedias, atlases, almanacs-- you get the picture) are a great way to both get the basics on your topic AND situate it within a broader context. Here are a few suggestions to get you started:
Guys, the library is cuckoo for reference sources, and you should be, too. Check out all the options here! Pro tip: get your angle from Credo or Gale, then go deep into the subject-specific sources (bonus points for alliteration).
Everyone's a Critic: Finding and Using Critical Commentary
Now we're getting serious. All the literary criticism, all the time. A simple title search should get you started. If you already have a theme in mind, you can add in that term, eg "Rime of the Ancient Mariner" and "repetition."