1968’s Spartan Planet is the fifth book in A. Bertram Chandler’s John Grimes series.
Policeman Brasidus and his fellow Spartans are quite familiar with human history. Men originated on Sparta. Like all the other animals on Sparta, men reproduce by budding, although medical technology has freed men from this unpleasant method of reproduction. Sparta’s society has been strictly regimented since the earliest days, because that’s the best way of doing things.
Although Sparta has very little use for advanced technology (which would only take jobs from the helots), there is a single extra-Spartan colony, Latterhaven. It was settled by rebels who maintain a monopoly on starflight. Not that it really matters, since the entirety of Sparta’s annual trade fits into two cargo ships, Latterhaven Hera and Latterhaven Venus. Aside from Sparta and Latterhaven, there are no other inhabited worlds and no races aside from Man.
A lot of what Brasidus and his fellow Spartans know is untrue.