Like many of Heinlein's works, this novel's lead character is a young man (to his fault Heinlein could not write women but to his credit before the end of his career he rarely tried) who makes a decision on a lark (joining the Mobile Infantry) and then learns about the universe and becomes a man. In the hands of a lesser writer, the novel would become a kind of interstellar version of British 19th Century gone to sea novels. Heinlein, however, uses it as a platform from which to engage in a biting and thoughtful commentary on the state of society in 1959.
Here we are offered an interesting theory of citizenship, the relationship between violence and political power, and the idea of rights. Interestingly, many assume this novel was written in the 1960s or 70s because of its apparent preoccupation with the idea of runaway crime and fear. Heinlein at the end of the staid 50s was prescient in what he saw as the coming threats to society. A rabid anti-communist, readers will also find interesting in the sort of society that Heinlein presents as the ideal opponent of a perfect communist system.
What makes this work exceptional is that all of these subjects are rolled neatly into a riveting read. I first read this novel in my early teens and it opened my eyes to a whole range of questions while still keeping me entertained. Controversy still rages among many about what the author intended with this book. Was he presenting an idealistic utopia of civic virtue or a militaristic dystopia bent on conquest? Read it and join an argument that will continue to rage for years to come!