The Algebraist

The Algebraist (2004) is a Hugo Award-nominated science fiction book by Iain M. Banks. This is not part of the beloved Culture series. It's a space opera with a culture all its own, with aliens and worlds you can escape into. I wouldn't suggest this for folks new to science fiction, but I'd feel comfortable recommending it to fans of space opera. It is a complex story.

from the back of the book:

The time is 4034 A.D. Fassin Taak, a Slow Seer at the Court of the Nasqueron Dwellers, will be fortunate if he makes it to the end of the year.

The Nasqueron Dwellers inhabit a gas giant on the outskirts of the galaxy, in a system awaiting its wormhole connection to the rest of civilization. In the meantime, they are dismissed as decadents living in a state of highly developed barbarism, hoarding data without order, hunting their own young and fighting pointless formal wars.

Seconded to a military-religious order he's barely heard of -part of the baroque hierarchy of the Mercatoria, the latest galactic hegemony- Fassin Taak has to travel again amongst the Dwellers. He is in search of a secret hidden for half a billion years.

But with each day that passes a war draws closer -a war that threatens to overwhelm everything and everyone he's ever known. As complex, turbulent, flamboyant and spectacular as the gas giant on which it is set, the new science fiction novel from Iain M. Banks is space opera on a truly epic scale.

Infinity Plus praised parts of it, but didn't seem to think those parts hung together well. SF Site says it "needed a ruthless pruning that it did not receive." The Guardian suggests the book's editor should've been more active.