Retired Colonel Nathan MacKinnie sits in a bar on Prince Samual's World, listening drunkenly to the equally-drunk boasting of three young Imperial Navy officers, representatives of an Empire whose overwhelming power put paid to MacKinnie's career as an officer for the city-state of Orleans, to say nothing of the city-state itself and its war with the Kingdom of Haven. On his way home from the bar, he and his former first sergeant Hal Stark are attacked and kidnapped by men who turn out to be agents of the Haven secret police, and the head of the secret police has a job offer for the former colonel, an offer to gain revenge on the Empire while buying freedom for Prince Samual's World...
This is the beginning of Jerry Pournelle's swashbuckling science fiction adventure novel A Spaceship For The King, first serialized in Analog from December 1971 through February 1972 and then published in paperback by DAW Books in 1973. Both the magazine and book covers featured art by Kelly Freas. A Spaceship For The King is set in the same universe as Pournelle's later collaboration with Larry Niven, The Mote In God's Eye. It is a period when the Second Empire is rebuilding on the ruins of the First Empire of Man, and conscripting all the worlds of man into the new Empire, will they or nill they. Not all those worlds are treated equally, as Haven's secret police have learned, and the fate of planets that haven't rediscovered spaceflight is to be colonies with no voice in their own future. This is as intolerable to MacKinnie as it is to Malcolm Dougal, the head of Haven's secret police, and Dougal has a plan. Haven is at a late 19th/early 20th century stage of technology, with steam cars and primitive radios, but a short distance away (in interstellar terms) is the planet Makassar, Makassar is even more primitive than Prince Samual's World, having been more heavily damaged during the Secession Wars that broke the First Empire, but in the capital city of Batav, there is an old subsector library, and Dougal hopes that the "trading" expedition he sends under MacKinnie's command will find something in that library that Haven's scientists and industrialists can use to put Haven into space.
The rest of the novel covers the expedition's journey to Makassar and the city of Jikar, where they find that their sponsors of the Imperial Trading Association have cleaned out the city and left nothing for them to trade. Publicly furious, MacKinnie and his team choose to remain on Makassar for a year, when the next Imperial ship will come by on its way to Prince Samual's World; privately, they begin working at once on a way to reach Batav. There follows a sea journey, a battle with pirates in which the traders use their superior ship and soldiering tactics to defeat pirates, and on arrival in Batav, MacKinnie and his people face the final challenge: can they somehow gain access to the "Holy Relics" of the city's Temple before the city falls to the plains barbarians? It's a marvelous adventure in an exotic land, but Pournelle also shows the squalor and misery of starving cities under siege, and the practical aspects of introducing slight technological improvements they hope the Imperial observers won't notice.
BUT WAIT! THERE'S MORE!
In 1980, a new version of the novel came out, titled King David's Spaceship. Whereas the first novel stopped at the point just after MacKinnie's army puts the Imperial Church's missionaries in control of Batav's theocracy and gains access to the old library/Holy Relics, the new version takes the story on to its logical conclusion. MacKinnie and most of his "traders" return to Prince Samual's World with the necessary information, only to find that even the primitive Mercury Project or the Soviet Union's Vostok rockets are beyond their ability to build. They are forced to adapt a pair of concepts stemming from fiction and untried theory to their Coal Age technology in a daring attempt to finesse the Imperial classification laws. Haven wins its freedom, but there's a price to be paid before the story's over. The expanded story is less of an adventure and more of a techno-thriller, as Haven desperately tries to complete its spaceship while avoiding the attention of the Imperials, even as it drags its feet on global unification to buy time, but it's a decent ending to the original story, which had some rather large loose ends. I recommend both versions.