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:
Search hundreds of reference sources simultaneously. Use the narrowing tool on the side to winnow down the possibilities-- the "Subject" feature is particularly useful.
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
A great starting point for accessible material. Often, a search of the title may be enough, as with "Goblin Market." Too many results? Try adding the term "criticism" and searching again.
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."
The granddaddy of humanities databases, JSTOR don't play. Combing through results can be a beast, but with all the great commentary and full-text documents, there can be big payoff.