A Fire Upon the Deep by Vernor Vinge

Millennia hence, an unknown force has partitioned space into ‘zones of thought’, which dictate a minds potential – from superintelligence in the Transcend, to the limited minds of the Unthinking Depths. When an ancient Transcendent artefact is used as a weapon, an awesome destructive power is unleashed. Feeling this threat, a family of scientists is captured by an alien race with a harsh medieval culture. A rescue party is assembled to retrieve them – and a secret that may save the rest of interstellar civilisation.