The Eagle: Rosemary Sutcliff

In 117 A.D. Flavius Aquila led five thousand soldiers of the Ninth Legion on a mission beyond the edge of the known world. They and their treasured Eagle standard were never seen again.
Twenty years later, the commander's son Marcus sets out to solve the mystery of their disappearance - and to restore his father's honour. This is the Story of his epic journey into the unknown north of Britain, on a quest so dangerous that no one expects him to return.