Stranger in a Strange Land is probably the most famous of all of Heinlein's works, known even to people who don't really read science fiction. It was picked up by the 70s rebellion culture of free love and anti-establishment sentiments and thus came into common usage. This book has been reviewed to death, so I'm just going to lay out some of my impressions, after many readings over the decades.
Of course, the entire idea of the story came from the germ of Heinlein wondering to himself, "What if there once was a Martian named Smith?". The novel marks a bit of a turning point in his career, where he really began to focus on writing to and for adults about very contentious issues, like religion, sex, crime and punishment, social responsibility, and what it really means to be human.
In synopsis, a mixed-gender expedition lands on Mars, and all hands are reported lost. When the next expedition arrives, they find one survivor, Michael Valentine Smith, who is the son of the captain and the ship's doctor's wife, and who has - in the time lapse between expeditions - been raised by Martians, who have an ancient culture far different from our own. He is returned to Earth, and spends hundreds of pages learning how to be human. Once he figures it out, he spends the last hundred pages of the book trying to teach humans to escape the shame and misery of the human condition, using ways of thinking he learned on Mars.
Heinlein really fully realizes in Stranger an gimmick he used quite heavily in later novels, introducing an "elder" figure - in this case Jubal Harshaw, retired lawyer and active author - who can deliver the lectures that Heinlein needs spoken, on whatever topic Heinlein feels we need to understand.
On free love...Heinlein had had an open marriage for years, at this point (never mind that his first wife went nuts) and uses Jubal's preaching and Mike's boyish innocence about Earth people's reproductive habits to expound on all of the reasons why everyone has an unlimited supply of love to go around, and shouldn't feel any sense of shame about their bodies or their desires or with how many or what types of partners they want to share the physical act. Ace reporter Ben Caxton bears some of the brunt of Jubal's criticism for allowing the green monster of jealousy and the ugly sin of hypocrisy to keep him from enjoying to the utmost the sexual freedom that Mike and his groupies (did I mention he starts a church?) freely practice.
I think we've managed to reap the results of the sexual revolution today, with its broken families, abandoned children, the genocide we call abortion on demand, and its degrading of social mores down to the most basic animal levels. It all looks great on paper, if you're a self-enlightened God on Earth, as Mike and his friends become.
Speaking of Mike's god-like powers, this is another theme that Heinlein was fascinated with for years, and occasionally tossed into the mix. ESP, telekinesis, precognition, and all of the mystical tricks of the mind are explained away in this novel when it turns out that we're all capable of miracles, if only we learn the proper way of using our entire brains - Mike learned it from the Martians.
There are some positive virtues that Jubal/Heinlein espouses, as well. Heinlein was always deeply patriotic, and in a future world that has now fallen under the rule of the United Nations, you still get the feeling that the U.S. of A is the best, most free place to live, and Harshaw demands that the President of the United States be given honor above all other nations by seating him at the head table with Mike when he is acting as the Martian ambassador. Jubal also delivers a great lecture about how to appreciate really good art, especially sculpture, with some of Rodin's pieces explained.
Heinlein's libertarian streak takes an interesting turn here. As the story begins to spool up, Jubal Harshaw is living in a seclusion - he's made enough money to build his "castle" and he doesn't have to put up with any nonsense from anyone. He obeys the laws he needs to obey (renders unto Caesar only what he must) and ignores the rest. He is a "man of good will", who cannot be bought, cannot be coerced, cannot be enslaved. Then the Man from Mars comes along and stirs up a storm in Jubal's household, wakening the old man from his slumbers, forcing him to take notice of a world that has for the most part, passed him by. I wonder if some other part of Heinlein's psyche was closely involved with that feeling; he lived in a small house in L.A. around this time, and was making a decent living from his writing, had no great cares nor obligations.
As the novel unfolds, Mike himself displays the ultimate in libertarian thought. He is a man who is totally in control of himself and his surroundings, he can't be physically attacked unless he allows it - his telepathy warns him of ill intentions, his telekinesis can block attacks, and if you really get on his nerves, he can disappear you into the fourth dimension. He can teleport away from any jail, survive naked in any environment, and make all the money in the stock market he needs - precognition, you know. His sense of morality and ethics comes from within, and it is rock-solid and perfectly good (maybe not from a judeo-christian perspective, but close enough). He needs no government, yet harms no one.
There's much more of interest I could talk about, but I'll leave something for you to discover on your own.
Personally, I think that Heinlein was very nearly at his peak at this point. The writing is interesting, engrossing, entertaining. There are only a few really good novels left in him at this point, before things started going downhill. It's grown a bit dated, now, but is still a fun read and a truly classic bit of SF.