Fellow English major graduate (from a few years ago) here. I had this book I checked out from the public library, the Encyclopedia of Allegorical Literature. It really helped me expand my horizons to different allegories when I had little time to read other materials. It also helped me see common allegories authors used!
https://www.amazon.com/dp/0874367816/ref=cm_sw_r_cp_apa_fabc_JF4Q90J5NFDEFCKH63AY