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Handouts are available in the library, and online support is terrific.
Your VERY best bet for critical essays and overviews of works. Start your search with the author, Ralph Ellison. Here, you can limit by Ellison as the subject, by type of source (biography, overview, or critical essay), and by title of work, Invisible Man. From within each search, you can also limit your results using key words such as "symbolism." Remember, Gale resources can be directly downloaded into Google Drive, and the citations can be exported directly into Noodle Tools.
You can always try to locate additional academic articles using Google scholar. However, you are often directed to subscription only databases like our Literature Resource Center, which you can't access without $, or to ebooks that you can buy. Please avoid non-academic sites like Weebly, Prezi, and the like as this tends to be student-generated content. Spark Notes is also not very academic, and while Schmoop is better, it tends to be simplistic.