Ultima Thule by Mack Reynolds
This novella is available free online:
premise of this novella is that humanity's expansion out to the stars was often
motivated by groups of people who wanted their own planet for their chosen form
of political, economic or religious society. As a result, the various planets
have radically different cultures which would see each other as misguided. They
are held together in a confederation with the number one rule of
non-interference with each other.
Ronny is hired as a new agent of a United Planets agency which is supposed to ensure this non-interference. He is assigned to track down a man, Tommy Paine, who apparently travels from planet to planet stirring up trouble. He is given an assistant who has the annoying habit of disagreeing with every opinion he expresses about any of the cultures they encounter.
Their first stop, Delos, is a planet with a theocracy where the leader has just been assassinated. The case is somewhat analogous to a locked-door mystery. The assassin used a bomb that could not have been made on that planet. However, the only outsiders that have been permitted on the planet have been United Planets employees - and none of them fit the profile for Paine. What’s more, the theocracy is very strict about monitoring both its own people and outsiders. They have the latest satellite detection system which shows that no unauthorized spaceship landed on or left from the planet. The only ship that has left the planet since the assassination was a United Planets craft.
Ronny takes a ship to follow the one that left after the assassination, but is unable to find any evidence that any of those people could be Paine or could have taken part in the assassination.
Next, he is sent to Kropotkin, an anarchist colony, mostly living at a 19th century standard of living. It seems someone is undermining their quiet, slow society by giving some people advanced technology Kropotkin would be unable to produce with its current level of industry and would be unable to purchase with its current system of trade.
Ronny begins to develop a theory about Paine, which is reinforced when he overhears a conversation. The solution leads him to a new perspective.
It's an interesting view of an interstellar civilization with a wide diversity of member planets and the confederation to which they belong.
The writing style is decent, but not great. There's also something about it that gives the story an older feel to it.