Sometime about A.D. 117, the Ninth Legion, which was stationed at Eburacum, where York now stands, marched north, and was never heard of again.
— Excerpt from the introduction
The Rosemary Sutcliff historical novel published in 1954 about a young Roman soldier named Marcus Flavius Aquila who goes on a quest to find the eagle standard of his father. It is loosely based on the mystery of the real Roman Ninth Legion, which disappears from the historical record after having last been mentioned as present in Scotland in the early 2nd century. This book sets up later novels featuring descendents of the protagonist. The 2011 film The Eagle is based upon this novel, but the earlier (1977) BBC TV series (titled The Eagle of the Ninth) is a more faithful adaption (perhaps because the 2011 film omits some characters and events for the sake of brevity).
This book (and the 1977 TV series) provide examples of:
All-Natural Snake Oil: Tradui insists toad fat can cure eye infections, even after Marcus' salve actually cures the poor baby who's been receiving this treatment.
Animal Motifs: An overarching theme in the book is to compare the freedom of the Celts with the servitude of Rome - the Britons beyond the wall are wolves and hunters, while Romans and their slaves in particular are domesticated hounds.
Marcus Aquila is associated with various birds. Not only is his cognomen Latin for eagle, but his childhood is regularly symbolised by an olive-wood bird he carved one summer, and he planned to spend his adulthood marching under the Eagles.
Cottia is nicknamed "the little vixen" for her red hair, sharp features, sharper temper, and tendency to bite.
the tribe that captured the Eagle are known as the Seal tribe.
To prepare for their foray beyond the wall, Esca grows a moustache, and Marcus grows a beard. Both men's facial hair grows out even more the longer they spend outside of "civilisation", and by the end are really glad to be rid of them.
Guern also has a beard when he first appears, though he ditches it in favour of a moustache.
Bittersweet Ending: Marcus has brought the eagle back, but the Ninth Legion cannot be restored.
Blue Blood: Placidus, who has a bit of a case of Aristocrats Are Evil, and technically Marcus, too, who is said to be of the Equestrian class, a Roman knight.
Call to Agriculture: Marcus' ultimate dream throughout the book is to buy back his father's farm and live out the rest of his days there after a successful military career. After his adventures, developing a love for the British landscape, and realising he sees his ancestral farm through a Nostalgia Filter, he instead sets up a farm in Britain with Esca and Cottia. Their farm is still there a century later.
Fashion Dissonance: Cottia's mum Valaria is obsessed with appearing as Roman as possible, and she wears a fashionable outfit to the Gladiator Games to this end. Thing is, not only is her outfit far too light for the British climate, but, as Marcus notes, it has actually gone out of fashion a year ago.
Fiery Redhead: Cottia. She's flame-haired, rebellious, and tends to bite.
Finish Him!: The amphitheatre crowd tries to do this to Esca, but Marcus and his uncle intervene.
Food Porn: Certain items, like Guern's venison and Sasstica's honey-cakes, are described with loving detail.
Important Haircut: Guern's shave when he returns from the hunt. As Marcus watches him, he notes just how similar and ritualised shaving is throughout the world, and realises that he and the Celts are Not So Different after all. subverted when we find out that Guern was in fact a Roman legionary once.
Jerk Ass: Tribune Servius Placidus. He tends to, and apparently enjoys, making other people feel small.
Large Ham: Marcus as Demetrius of Alexandria. Esca has to physically stop him from talking once he starts haranguing the soldiers at the Wall.
Maybe Magic, Maybe Mundane: While Marcus is trying to steal the eagle, his lamp nearly goes out, but recovers when he prays to Mithras. Was it just a fault in the wick, or were supernatural forces at work?
Shadow Archetype: Guern to Marcus' father. Both were officers in the Ninth Legion Hispana, but Guern was a deserter who had lost faith in the legions while Marcus' father died a hero's death fighting for them.
Signature Item Clue: While staying with the people who took the titular eagle from his father's legion, Marcus removes the eagle during the night and hides it under the banks on the edge of a lake, for Esca to return in secret and retrieve days later, after the two of them have been followed and searched. Unfortunately, while doing so, Esca accidentally drops Marcus' distinctive ring-brooch, already "all but torn out" of Marcus' cloak through the roughness of the aforementioned search. When the natives find it days later, they give chase.
Snake Oil Salesman: Marcus disguises himself as an eye-doctor to get through Scotland safely. Subverted in that the eye-medicine he sells actually works, he performs his services pro-bono so as not to get weighed down by expensive trinkets, and he sometimes has to undo the further damage tribal medicines have wrought.
Stupid Sexy Flanders: Marcus and Esca have their moments when Placidus is around; Marcus especially when he lovingly describes Placidus's appearance.
Esca: He was a splendid young Tribune, smooth as a girl!
Marcus: ...an extremely beautiful young man, with the graceful carriage, oval face, and clustering hair that suggested Athenian ancestry.
The Wall Around the World: Hadrian's Wall is treated as such - as the boundary between Roman-occupied Britain and barbarian Scotland, it marks the start of Marcus' adventure and the place where civilisation ends from the Roman perspective.
Throwing Off the Disability: Downplayed - after receiving relatively decent medical attention, Marcus can walk again, but to his despair he'll never be fit for active service again. While he initially has no problems walking or riding through Scotland, his injury starts to get the better of him when he's forced to flee.
Unspoken Plan Guarantee: Marcus' plan to take the eagle without attracting suspicion is described in full, and is thwarted by Esca's dropping of the brooch. How the pair ultimately succeed is, by contrast, not described beforehand.