The Divine Comedy
Otherwise known as the source of Dante's Inferno, the one everyone knows, but also includes, as far as the whole cycle goes, Inferno, Purgatory, and Paradise. When I was studying this one in high school, we actually watched Bill & Ted's Bogus Journey (second most awesome school experience, after a college class clip from Life of Brian), as a comparison. It's one of those classics works of literature people barely know but have at least heard of.
Bookshelf status: probably needs a reread.