The Eagle of the Ninth, Rosemary Sutcliff, Farrar Straus Giroux.

In A.D. 119 the Ninth Roman Legion marched north into the wilds of Britain beyond Agricola's Wall and disappeared without a trace. Fifteen years later, Marcus Flavius Aquila, the son of the unit's commander, embarks on a quest to recovers the lost eagle standard on the Ninth, symbol of a legion's- and his family's- honor.