I am now re-reading "Pan Lodowego Ogrodu" by Jarosław Grzędowicz. (There are three published tomes, but sequels are in writing)
In english it would be like "The Lord of the Ice Garden". But believe me, it sounds better in Polish.
And I must say, it's one of the best things I have ever readed. Polish is complex, right, but reading Polish books, especially from native writers, it's a far, far better experience than reading English ones.
The plot of "PLO" is set in the future - people can now travel to other solar systems without any problem. And after years of space exploration they find something unique - other civilization. However, even if the natives look and behave just like humans, they are stuck in Middle Ages-like era.
Several groups of humans are sent to the planet to examine everything - natives' culture, flora, fauna, stuff like that. Also, a rule is set - no human is allowed to in any way disturb the culture and reveal himself.
But after some time, one of the scientific groups get lost. They literally disappear, not leaving any message or trace.
Unfortunately, the Rule says that sending an army there is not an option. So, one single soldier is send in a rescue mission - a soldier with best possible training and with biological implant that gives him unbelievable physical and mental possibilities.
But even that isn't enough to prepare him to what he will see in that world...
World in which, apparently, magic DO exist.
So, this book is, in fact, an science-fiction and fantasy in the same time.
Believe me, you can't NOT love this book. Great ideas, ultrarealistic characters and one of the best main protagonists of all the time. If anyone would ever get to learn Polish , know this - these books alone are enough reason to learn this language.