Lavinia by Ursula K. Le Guin is temporarily marked down to $3.99 in ebook format on Amazon. This one takes a character from The Aeneid and develops her story. It is very well-reviewed.
Review from Publishers Weekly since a book description was missing:
In the Aeneid, the only notable lines Virgil devotes to Aeneas' second wife, Lavinia, concern an omen: the day before Aeneus lands in Latinum, Lavinia's hair is veiled by a ghost fire, presaging war. Le Guin's masterful novel gives a voice to Lavinia, the daughter of King Latinus and Queen Amata, who rule Latinum in the era before the founding of Rome. Amata lost her sons to a childhood sickness and has since become slightly mad. She is fixated on marrying Lavinia to Amata's nephew, Turnus, the king of neighboring Rutuli. It's a good match, and Turnus is handsome, but Lavinia is reluctant. Following the words of an oracle, King Latinus announces that Lavinia will marry Aeneas, a newly landed stranger from Troy; the news provokes Amata, the farmers of Latinum, and Turnus, who starts a civil war. Le Guin is famous for creating alternative worlds (as in Left Hand of Darkness), and she approaches Lavinia's world, from which Western civilization took its course, as unique and strange as any fantasy. It's a novel that deserves to be ranked with Robert Graves's I, Claudius.