Pretty much everyone knows the basic outlines of the Odyssey and the Iliad; I believe that the Aeneid has been rather lost in the last few decades. For those who don't know it, it is a long poem in twelve books by the great Latin writer Virgil, recounting the tale of the Trojan prince Aeneas and his escape from Troy to found a settlement on the future site of Rome, despite the temptations of Dido, the queen of Carthage, and various other setbacks along the way.
Myself, I did the first half of Book II for my Latin O-level many years ago (a lengthy flashback where Aeneas recounts the fall of Troy) and had skimmed to the end of Book VI in translation, but I realised reading Lavinia that I had never even started the second half. And the great thing is that it doesn't really matter; it is striking that the book appears to appeal to readers who don't know Virgil at all as much as to those who know him backwards.
This is partly because Le Guin introduces Virgil himself into the book as a character, a ghost from the future trying to finish his poem, discussing it with Lavinia, filling her in on the bit of Aeneas' story she hasn't experienced herself, aware some how that he himself is going to feature in someone else's poem, and making her aware that she is in fact a character in his. It's a profound reflection on Story and what it means to those who tell it, and those who are in it.
The other fascinating characters are Aeneas and his destiny. Aeneas, rather like Frodo, has a quest to follow and fulfill, and is grimly conscious of that burden (which loses him the first two women he loves, his Trojan wife Creusa and Dido of Carthage). Virgil likes to describe him is 'pious', which has all kinds of confusing connotations for today's audience. Le Guin unpacks this infuriating adjective and explains Aeneas to us much better than any translation could.
On top of that, the world-building is super. Lavinia's pre-Roman Latium is pagan, of course, but does not have the anthropomorphised gods that Virgil knows. It is also slightly magical - omens come true; Aeneas' shield tells the future; and of course Virgil himself appears, possibly more real than Lavinia. The social structures of gender and power are beautifully delineated. I have no hesitation in voting Lavinia top of my bsfa ballot, and share the dismay of those who wonder why it did not greater recognition for the Hugos or Nebulas last year.
(See also an excellent group discussion of the book in four parts: 1, 2, 3, 4.)