Edition: Penguin, 1981
Review number: 321
At one stage of his career, Heinlein wrote a series of novels aimed at what is now termed the "young adult" market; Citizen of the Galaxy is one of the best of these. This is partly because it has something of a message yet is still entertaining escapism.
The moral is hardly a revolutionary one; it has been pretty generally accepted throughout the twentieth century. It can be summed up as "slavery is evil", and though mainly concerned with slavery as traditionally practised, it contains rather subtler references to extend the idea of slavery to cover any life determined by involuntary rules imposed from outside. The central character, Thorby, basically passes through several different sorts of 'slavery' - ownership by another, membership of a ritually constrained culture, the discipline imposed by the armed forces. (I am not sure that Heinlein would have considered this a form of slavery when enlistment is voluntary.)
The plot of the novel is fairly implausible. Thorby, a slave for as long as he can remember, is sold at the market on the planet Sargon VIII, part of a group of planets (the Imperium) where slavery is an important social institution. He is bought - at a knock down price - by an old beggar, Baslim, and adopted as his son. Baslim is not what he appears to be; he turns out to be a spy dedicated to eradicating slavery. When suspected by the Imperium he is killed; but Thorby escapes with the help of the Free Traders Baslim had used as couriers for his messages.
The Free Traders are basically spacegoing merchant princes, who have subordinated their entire lives to ritual designed to maximise profit and make it possible for a large community to live in the restricted environment of a spaceship. When Thorby finds this new slavery constricting, he is able to join the armed forces of the Terran Hegemony, in which Baslim had been an important officer. The identity check performed on new recruits then causes the most implausible twist in the book: Thorby is actually Thor Rudbek, heir to one of Earth's biggest corporate fortunes, presumed dead after his parents' space yacht went missing when he was a baby.
The implausibility is hidden by skilful writing; the characters are believable' and this makes it one of Heinlein's best books.