I came into this book having read two of Heinlen's kiddy books, The Star Beast and Time for the Stars. The difference is probably akin to that between the D-League and the NBA. That is, a lot more sex happens. In short, the book parallels The Jungle Book. But instead of a boy raised by monkeys, we get a man raised by Martians. The novel covers, in five parts, his background story, his adjustment to Earth, and his formation of a pseudo-religious super-sexual cult-like organization. In between, there's a lot of other stuff going on, like politics and rants from Jubal Harshaw. I initially thought (from just briefly seeing the word Mars on the cover and whatnot) that the book would be about Martians and space travel and all that jazz. It was not. Instead, it was a sociopolitical take on humanity, on how we live and treat each other, and on how we might find ways towards more happiness/pleasure in some of our practices. Here are some quotes I found interesting, which are mostly about sex. I've left out the misogynistic ones; Goodreads has that area covered pretty well.
Her answer was not in words. Then, as their grokking made them ever closer and Mike felt himself almost ready to discorporate her voice called him back: "Oh!...Oh! Thou art God!" (266)
This is the first time Mike and Jill "join." The language used during scenes of this type doesn't deviate too much from the norm. In fact, this quote packs in a bunch of Heinlen-specific words/phrases: grok, discorporate, "thou art god." I was a bit weirded out by this - perhaps not by the mere prescence of the language itself, but by the magnitude it gave the associated actions. Here are some other slightly less explicit quotes.
But Jill did not know that Patricia had met a holy man before - she expected more of holy men. Jill was serenely happy that a cusp had been met with right action... then was ecastatically happy to grow closer herself. (285)
"Naughty pictures are a great goodness," Mike said gravely. (307)
Jubal cursed and reached for her... and cooperated with the inevitable. (408).
Some other random quotes.
...turned down everybody from Cal-Tech to Kaiser Wilhelm Institute in the past; Harvard could not resist the bait. (322)
An artist can look at a pretty girl and see the old woman she will become. A better artist can look at an old woman and see the pretty girl she used to be. A great artist can look at an old woman, portray her exactly as she is... and force the viewer to see the pretty girl she used to be... more than that, he can make anyone with the sensitivity of an armadillo see that this lovely young girl is still alive, prisoned inside her ruined body. He can make you feel the quiet, endless tragedy that there was never a girl born who ever grew older than eighteen in her heart... no matter what the merciless hours have done. (323)
Overall, I would recommend this book. In both content and style, it differs greatly from most modern books, making it interesting in novelty alone. Besides that, it's just a good read: entertaining, funny, and sometimes eye-opening. At the very least, you can grok a bit more once you're done.