This is probably the best known of all Heinlein's novels as it reached an audience far beyond the science fiction fans who normally picked up his books. It became so influential that in 2012, the US Library of Congress named it one of 88 "Books that Shaped America". Set in a universe related to the one in his juvenile Red Planet(the Martians are clearly the same), it tells the story of Michael Valentine Smith, a child of two of the astronauts on a Mars expedition. He was born on the spacecraft, and is the only survivor of the expedition. Raised by Martians, he eventually is picked up by another expedition. Since he knows nothing of Earth, it gives Heinlein a chance to see our culture from the outside to some degree. Published in 1961, it became a text for hippies a few years later (some of whom made a "pilgrimage" to Heinlein's home, which he did not encourage) because of its emphasis on free love and critique of religion. Other ideas include the notion of "water brothers", and the introduction to the language of the verb "grok", a supposedly Martian word Heinlein made up. It won the Hugo award, and was the first science fiction novel to enter the New York Times Book Review's best seller list.

The real protagonist of the book is the older man Jubal Harshaw, and most people think this is Heinlein portraying himself to a significant degree. Harshaw is knowledgable guide for Smith, a loco parentis, and allows Heinlein to deliver pronouncements on the state of society, something he became more prone to do as he got older. I think this is a transitional novel, therefore, that takes us from the more hard science fictional writings of the early Heinlein to the more preachy works of the end of his career. That does not mean there is no science fiction in this work, or that there was no preachiness in earlier works. It is more a matter of emphasis. This is also the first of his novels where sex becomes a preoccupation, as it would remain for the rest of his career. Some people have said this is the beginning of Heinlein's "dirty old man" phase, therefore. But in any case, Heinlein is always a great writer, and I have read all of his works multiple times.