If you're into sci-fi I urge you to read Vernor Vinge's A Fire Upon the Deep [1]. It doesn't have a lot to do with this except that it has a fairly primitive society that has high-tech dumped on it in two different camps and what eventuates is an arms race.
In the same vein is CJ Cherryh's Foreigner series [2], which tells the story of human colonists who end up lost, unable to find their way home. They instead find a habitable planet with a native population that roughly has 19th century technology and build their space station above it. The station isn't sustainable though so they end up getting stranded on the planet.
So while not time travel per se, the whole subject of vast technology differentials is a rich vein of sci-fi that I personally really enjoy.
As for someone transported to a medieval era, any person with a high school education would have what is, for the time, a highly advanced knowledge of mathematics, astronomy, physics, possibly chemistry, biology, etc to the point where they would be immensely valuable in the right hands I would think.
You dont need sci-fi to ponder what happens when a superior technological culture meets a less technologicaly developed culture.
Look at Europes "discovery" of the americas, and for that matter, look at how the nomads of the world react when they are even today being "discovered" by us civilized people. You could study anthropolgy if you are into this sort of thing, its not really sci-fi, this is the real deal.
Which me of another of my favourite books, Iain M. Banks' Excession [1], which is pertinent to your example. It's a great book but is probably best known for Outside Context Problem, which is the kind of problem which:
> ...most civilisations would encounter just once, and which they tended to encounter rather in the same way a sentence encountered a full stop.
Also:
> The usual example given to illustrate an Outside Context Problem was imagining you were a tribe on a largish, fertile island; you'd tamed the land, invented the wheel or writing or whatever, the neighbours were cooperative or enslaved but at any rate peaceful and you were busy raising temples to yourself with all the excess productive capacity you had, you were in a position of near-absolute power and control which your hallowed ancestors could hardly have dreamed of and the whole situation was just running along nicely like a canoe on wet grass... when suddenly this bristling lump of iron appears sailless and trailing steam in the bay and these guys carrying long funny-looking sticks come ashore and announce you've just been discovered, you're all subjects of the Emperor now, he's keen on presents called tax and these bright-eyed holy men would like a word with your priests.
The entry also notes the similarity to black swan theory [2].
In the Foreigner universe it is my understanding that the technology was not 19th century. It reached 19th century after centuries of careful technology transfer. (And then things accelerated.)
In the same vein is CJ Cherryh's Foreigner series [2], which tells the story of human colonists who end up lost, unable to find their way home. They instead find a habitable planet with a native population that roughly has 19th century technology and build their space station above it. The station isn't sustainable though so they end up getting stranded on the planet.
So while not time travel per se, the whole subject of vast technology differentials is a rich vein of sci-fi that I personally really enjoy.
As for someone transported to a medieval era, any person with a high school education would have what is, for the time, a highly advanced knowledge of mathematics, astronomy, physics, possibly chemistry, biology, etc to the point where they would be immensely valuable in the right hands I would think.
[1]: http://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep
[2]: http://en.wikipedia.org/wiki/Foreigner_universe