Just two years after producing Starship Troopers, a book beloved of right wing militarists everywhere, Robert Heinlein came up with Stranger in a Strange Land, in which guns are seen as "a great wrongness", personal and sexual liberation a "goodness", monotheistic religion no better than a carnival trick and making money an absurd diversion from the real business of life.

It's debatable whether this contrast is a testament to the variety of Heinlein's talents, the complexity of his politics or the dangers of taking anything written in a novel as an indication of a writer's own viewpoint. What is certain is that in its own way, Stranger In A Strange Land is just as effective as Starship Troopers. It might not be, as paperback covers declared it for years, "the most famous science fiction novel of all time", but having sold more than 5m copies, it's certainly among the most influential. Its enthusiastic promotion of free love made it a staple of the 1960s hippy movement. It gave birth to a word that entered common counter-cultural parlance ("grok", meaning, roughly, to understand fully and with empathy). Most notably of all it inspired a religion — The Church Of All Worlds — which survives to this day.

These achievements seem all the more impressive given that Heinlein pumped out all 222,000 words of the thing in just two months. It was too long, too controversial, and too damn strange for Heinlein's editor, who lopped 60,000 words off before its publication in 1961. Many consider Heinlein's original, published uncut in 1991, to be the better book, but I went for the shorter version — after all, that's the one that won the Hugo, and I figured there was only so much I could take. By the time I got to the end of Stranger, I'd certainly had my fill of bizarre-SF-sauciness and libertarian philosophy. But that doesn't mean I didn't enjoy it.

The book begins like a kind of extra-terrestrial Jungle Book. It's the story of Valentine Michael Smith, a human raised on Mars — by Martians — who finds planet Earth entirely alien when he first arrives there. Heinlein has great fun pointing out human oddities and foibles as Michael tries to "grok" the world around him, while creating an enjoyably nasty future world governed by backroom deals, black propaganda, and (decades before Nancy Reagan) the astrological obsession of the wife of the nominal head of state. There are plenty of tense moments as Michael and the people who befriend him escape and outwit various "security" goons and there's plenty of intrigue as a political case is put forward for his survival. Sure, there are hints of things to come in the form of busty nurses and beautiful secretaries frolicking in swimming pools, but this part of the book is relatively straight.

It's only once he's got you hooked that Heinlein really lets the weirdness flow. In the second half, Michael establishes his own religion based on his experiences on Mars, combined with a conviction that clothes are essentially pointless and that having sex with lots of people is a very healthy way of getting "closer" to them.

Mercifully, Heinlein doesn't indulge himself much beyond a few lascivious hints about hot tubs, long kisses so powerful they make women faint, clothes magically disappearing and characters feeling wonderful the following morning (at least, not in the version I read). All the same, it's easy to see why Jess Crispin wrote:

"If this is what SF fans hold up as a classic, no wonder the outside world thinks the geeks are all a bunch of loonies. Can't we have a classic that doesn't have orgies? Can we agree that Heinlein writes about free love and fascistic governments and pick another representative for the genre? Because this is obviously not working … "

But that's just the kind adverse reaction Heinlein expected. He said he wanted to slaughter "the biggest, fattest sacred cows" of western society — chief among which is monogamy. Certainly the challenge Michael presents to conventional mores is provocative — and must have seemed especially so in 1961 — and it's all presented in an appealingly gentle way (with the exception of the fact that Jill, one of the lead characters, is prone to homophobia and saying things like: "nine times out of ten, if a girl gets raped it's partly her fault"). The lovey-dovey joy of Michael and his followers also provides an effective contrast with the more sharply barbed satires on religion and the media.

So yes, it's daft, but it's also thoughtful and generally good fun. Heinlein does have a tendency to lecture and harangue, but generally does an excellent job of making palatable some challenging ideas. You have to be a particular type of person to take these ideas entirely seriously, but as "an invitation to think — not to believe" (as the author once described it) Stranger From A Strange Land is a resounding success.

Next time: The Man In The High Castle – Philip K Dick