The Return of the Emperor by Allan Cole and Chris Bunch

The Emperor is dead, killed by an assassin's bullet, and all evidence destroyed by a small nuclear blast immediately following his death. The privy council has taken over all administrative duties for the Empire, but they are somewhat hamstrung by the cessation of all AM2 supplies - the mysterious substance that the Emperor once controlled is necessary to provide power for all space travel, manufacturing, and other industries. The Empire has fallen into a universal depression, and may descend into a Long Night of isolation and barbarism, if something is not done soon.

Ian Mahoney has faked his own death and gone into hiding, as the privy council's minions have been quietly gathering up and interrogating anyone who was close to the Emperor, hoping to learn the secret to turning the AM2 flow back on. When the council's goons go after Alex, on his home world of Edinburgh, and Sten, on the tiny planet of Smallbridge, Ian contacts them, and they flee to rendezvous at a safe, secret location to confer. Ian has been doing some secret research and been convinced of two things: the privy council is responsible for the death of the Emperor, and the Eternal Emperor has been killed more than once before, and returned within two years to take over his empire once again. This time, though, it's been six years, and the Emperor has not appeared.

Sten and Alex recruit some old friends from Mantis and the military establishment who are also convinced of the privy council's guilt, and they launch a disastrous attempt to capture or kill all of the council members at a gathering on Earth. When this fails, the duo flee to the Wolf Worlds and take refuge with their old friends, the Bohr.
Eventually, they decide the route to justice must take a more official-looking path, and organize a Tribunal to investigate, evaluate and issue an indictment against the council. Sten and Alex get the dirty job of stealthily gathering the evidence.

In the counterpoint tale, a human is awakened from cryogenic slumber on a ship located in another universe, and hypnotically educated/indoctrinated, then sent on his way to first a planet containing a mansion with a comprehensive library where he learns all about the events of the past six years, then to a spaceport where he displays an innate talent for cooking in a greasy spoon diner, and finally to the planet Dusable in the Cairennes system. On Dusable, he shows an amazing talent for political dirty fighting, and helps to get a new chief executive elected there. The account of the hands-on electioneering tactics is fantastic fun!

More great action, intrigue and a few minor mysteries cleared up in this installment.