Few authors can mix science fiction and fantasy elements together well, and of those who can, one of my favorites is Anne McCaffrey and her “Dragonriders of Pern” series. Seemingly set in a fantasy realm, with a highly structured and almost medieval society, Pern is a world threatened by a vicious terror known as Thread, which falls from the skies whenever the Red Star can be seen through the Dragonstones.
The Thread, thin silvery filaments that fall from the sky and seem to eat away at any living thing they touch, are fended off by the Dragonriders, who fly on their dragons, using flame breath to destroy it before it can touch the ground, consuming plant and animal alike.
However, as the books progress, people start to discover artifacts from past generations more advanced than what they currently have, slowly beginning to reveal the science fiction basis for the books.
However, in the way of progress are tradition and some elements of the complex social structure of Pern, ones that favor their own power and prestige over the prosperity of society as a whole.
At the same time, the protagonists must comprehend tough, pragmatic choices that while they will get the job done and likely produce the best end result, may be somewhat morally questionable. These can range from things as simple as political manipulation to setting up duels to the death.
All in all, I recommend this author and her works. Look her up in the fantasy and scifi aisles!