Consider Phlebas
The Culture's debut and British space opera's resurrection note; its galaxy-of-plenty-with-doubts framework set the agenda for the next three decades of the form.
Series Guide
The Culture
Banks's post-scarcity utopia of humans, drones and godlike ship Minds — and the morally compromised business of nudging less fortunate civilisations, handled by Contact and the ominously named Special Circumstances.
In the Guide
The Player of Games
Widely recommended as the ideal Culture entry point and a fixture of best-space-opera lists; its empire-as-game conceit is one of SF's perfect metaphors.
Use of Weapons
Routinely voted the best Culture novel; its interleaved-chronology structure (suggested by Ken MacLeod) is among the most influential formal gambits in modern space opera.