Sartor Resartus is the work most likely to appear on the GRE Literature Exam.
Sartor Resartus (Latin, meaning roughly "The Tailor Re-tailored" or "The Tailor Re-clothed") is intentionally difficult to classify: it flits between satire and serious philosophy, between fact and fiction. The word was published in Boston with a preface by Emerson, where the work had a major effect on Transcendentalism.
For the exam, it's usually enough to be able to recognize the following characters and keywords as belonging to Carlyle's Sartor Resartus:
-Diogenes Teufelsdröckh (Latin, "God-born," and German, "Devil-shit")
-The Wanderer (another name for Teufelsdröckh)
-The Everlasting Yea (roughly, spiritual faith)
-The Everlasting No (roughly, spiritual doubt or cynicism)
-Weissnichtwo (Teufelsdröckh's hometown; German, "know not where")
If you want to go deeper, I recommend the Oxford World's Classics Edition