The Sterkarm Handshake, by Susan Price (Scholastic, 1998).
Imagine that, round-about our present time here in the 21st century, capitalist entrepreneurs have discovered how to travel in time. The thought of all the natural resources back there in the past, waiting to be exploited, makes them happy.
One of the time tunnels they have constructed leads to the sixteenth century in the wild boarder lands between England and Scotland. The Sterkarm clan who rule the patch of this land are fierce, treacherous, loyal to each other and not giving a damn about anyone else, and they are cognizant that the time travelers have much to offer (the aspirin tablets are a hit).
Andrea is a young anthropologist, embedded back in time among the Sterkarms. Literally--she and Per, the son of the chief are passionately involved. For Andrea, deemed unattractively large by her own society, it is nice to be lusted after, and Per does genuinely care for her....it might even be love (although I couldn't help but wonder about how much her emotions were colored by her new desirability, and this made me uncomfortable).
But all is not well. The problem with greedy exploitation is that often the people being exploited fight back, and things go sour. The trouble in this case begin when Per, gravely wounded fighting off raiders (all in a days work for the Sterkarms), is taken by Andrea to the 21st century. The director of the company, a nasty piece of work, wants him as a hostage. Per escapes, makes his way through the tunnel home, and then he and his people declare war on the 21st century, burning what they can of the tunnel.
It is rebuilt, and the 21st century comes to make war in the past. It seems as though its an uneven match--heavy artillery against bows and arrows. But arrows can kill, and the Sterkarms have years of experience with treachery and guerrilla warfare...
So it basically stopped being fantasy neo-colonialism (interesting), and became a military sci fi story (not my cup of tea), and by the last hundred pages I was skimming because everyone was running around bashing each other etc., and I ws really tired of hearing about Andrea's predicament (torn between two conflicting loyalties, and not wanting any one to be killed, and not wanting the boy she's been sleeping with to be a ruthless killer even though he clearly is etc).
And did Andrea, intelligent anthropologist, save the day with intelligent anthropologizing? No. She went to pieces, and was all "Oh Per if you love me you will be kind and do something and not kill the people from the 21st century." Disappointing.
What it needed was more characterization and less fighting, in my opinion. The bad guy was one dimensional, and so uninterestingly bad that there was little point to him. Per and Andrea are two dimensional at best. In as much as they are already sharing a bed by the time we meet them, there is no subtlety to their relationship, and I never believed that they were actually in love with each other as people, as opposed to fond bedmates (I have nothing against affectionate lust enjoyed by both parties, but it's not as interesting as the tension of love being realized), and like I said, I didn't need Andrea's dilemma drummed into my head quite so much. A few minor characters come to interesting life, most notably Joe, a homeless Sterkarm descendant of modern times, who travels back to find a better life for himself--his is a fascinating little side-story. But this wasn't enough to actually make me care all that much.
Final thought--loved the premise, and thought the story was fascinating. If the book had been about 150 pages shorter, I might well have enjoyed it lots. As it was, it kind of oozed over the edges of its central story, and I lost interest.
However, don't necessarily take my word for it---The Sterkarm Handshake won the Guardian's Children's Prize, and got lots of critical acclaim, and is pretty much a classic of military/capitalist time-travel.