The Space-Time Juggler — John Brunner

In the long twilight if a galactic empire, the old king is dying. He has little choice but to name his callow young son as heir and his wanton daughter as regent. It seems the long decline is destined to continue.

But everything is changed by the appearance of a rival claimant: a long-lost princess, accompanied only by a loyal champion and a mysterious advisor. Will her arrival herald a bright new dawn for the empire? Or drive a once-proud civilisation to the brink of war . . . ?