The Algebraist is not a Culture novel, but it's sort of Culture-adjacent. Some particular differences are that there is a more structured government, and AIs are banned as abominations.

A mustache-twirlingly evil (and fortunately seldom on-stage) invader is leading a force into a star system, and the system has to prepare in various ways for it, including chasing after a probably mythical MacGuffin that was learned of, almost by accident, in a conversation with a representative of the gas giant-dwelling ancient species in the system. On the smaller scale, four friends go on a little exploration, and one dies. The remaining three bump into each other over the ensuing years, dealing with the overall situation and their interpersonal relationships. Sex, violence, a little puzzle, some whiz-bangery... it's a pretty satisfying mix, but I'm a little surprised it got a Hugo nod (though I certainly enjoyed it more than the winner, Jonathan Strange & Mr Norrell).