King David's spaceship1980

Four levels of technology collide in this fascinating sci-fi thriller about a Renaissance general racing to extract the secrets of space flight from a forgotten library guarded by barbarians on horseback before the bureaucrats of a re-ascendant galactic empire can finish classifying the Renaissance general's world as a flightless backwater planet fit only for subjugation and exploitation. As is often the case in hard sci-fi, the characters' personalities are neglected in favor of in-depth discussion of military tactics, historical speculation, and political economy, but the writing is tight, thoughtful, and fun, so it's not too hard to suspend your disbelief.

This book is a sort of sequel to the much more famous The Mote in God's Eye, but the events are largely independent.