The Algebraist by Iain M. Banks

For short-lived ‘quick’ races like humans, space is dominated by the complicated, grandiose Mercatoria, whose rule is both military and religious. To the Dwellers who may live billions of years, the galaxy consists of their gas-giant planets – the rest is debris.

Our human hero, Fassin Taak, is a Slow Seer privileged to work with the Dwellers of the gas-giant Nasqueron in his home system Ulubis. His work consists of rummaging for data in their vast, disorganised memories and libraries. Unfortunately, without knowing it, he’s come close to an ancient secret of unimaginable importance….