The Inferno, written by Dante, was primarily unique in that it is the first medieval book to be written in a vulgate language (Italian), instead of the far classier language of Latin. In it, Dante, led by Vergil (hmmm…wonder what epic Dante liked?) journeys through the nine levels of Hell. (He’s later led by Beatrice through Purgatory and Heaven in the sequels).
In Hell, Dante spies any number of mythical creatures, many of whom are fittingly from the underworld, as well as a great number of Greek heroes. For instance, Achilles (I never did figure out his logic, but a lot of it does have to do with medieval myth) and Dido are stuck in the second circle, for the lustful. Ulysses and Diomedes didn’t get so lucky—they’re in the eighth circle, a nasty place reserved for the treacherous.
Although not a classical book on the Hellenic Underworld,
the Inferno does give an idea as to how people in the medieval era saw
the afterlife and, for all that, the denizens of classical mythology.
(In fact, if you were good in the classical world, you got a much better
place in Dante’s hell than you did in the House of Hades. In the
Inferno, virtuous pagans get to go to Limbo, a place that’s like paradise
except that it lacks God. Sure sounds a lot better than being stuck in
a place where you’re just a pale reflection of your former self.)