The Court of a Thousand Suns by Allan Cole and Chris Bunch

Young Sten, having covered himself with (covert) glory in the affair of the Wolf Worlds, has been promoted to Captain and given an assignment as commander of the Eternal Emperor's bodyguard on Prime World. When a freedom fighter's meeting with the Emperor's ambassador is disrupted by a bomb, killing both of them and one innocent bystander, Sten is detached from the palace to act as a liason with the police. He and the lovely lieutenant Lisa Haines must unravel the plot, no matter how twisty the path.

The only one of Sten's old colleagues from Mantis section involved with the investigation, at first, is the inimitable Alex Kilgour, and we are once again treated to his shaggy dog stories. Sten and Alex track the small-time bomber who was hired for the job to a prison planet in the Tahn worlds, and impersonate prison guards in order to kidnap him and proceed with his interrogation.

After that link leads them to a renegade Mantis section doctor, they take a trip to a very dangerous mining world to find the next link in the chain, but the mad medic suicides before they can take him away, leaving them only with the mysterious clue, "Zaarah Walid". Eventually the trail leads deep into the heart of the palace itself, and reveals a dangerous secret from a decades old war.

Great fun, as always, in this series.