A Fire Upon the Deep by Vernor Vinge

The scientists in the High Lab, an expedition into the part of space known as the Transcend, have uncovered something very old, very powerful, and very dangerous. A glorified librarian, an unfrozen space pirate, and a pair of sentient seaweeds go on a desperate mission to a medieval world to try to stop it.

( It’s actually much less absurd than it sounds, relatively hard SF. I’m going to discuss spoilers in here, because I think everyone I know has read this already. )

Overall: This is a very good, very inventive book full of interesting ideas. It’s also damn long and occasionally confusing in its density. Recommended if you like sci-fi.