As a rule, essays about literature are written the present tense, especially when referring to the author..
"Shakespeare uses blank verse for nobility and prose for common people." "Pope's genius is responsible for many of the most common English adages." You can, however use other tenses if you are just recounting the story. "Hamlet knew he could defeat Laertes in a duel."
Essays about other topics can be written in the tense (tenses) that is (are) appropriate to the subject matter.