The pad of the male big toe applied to the clitoris or the vulva generally is a magnificent erotic instrument,” Alex Comfort, M.B., D.Sc. informs the carnally curious on page 50 of The Joy Of Sex, the book that shook the planet upon its publication in 1972 and remains the world’s most famous, and arguably most trusted, sex manual. “Use the toe in mammary or armpit intercourse or any time you are astride her, or sit facing as she lies or sits. Make sure the nail isn’t sharp. In a restaurant, one can surreptitiously remove a shoe and sock, reach over, and keep her in almost continuous orgasm with all four hands fully in view on the table top and no sign of contact… though she may appear more than a little distracted.”

An English-language classic if ever there was one – although perhaps for reasons different than the late Dr Comfort initially intended thirty-one years ago – this most, ahem, seminal of sex manuals was revised and reissued in a 30th Anniversary Edition in 2002 by his political journalist son, Nicholas. As of this month, it has been further modified into a slimline pocket edition, its lavender sleeve and slender dimensions suggestive of a consumer choice book: a wine buyers’ guide, perhaps.

Size (not mattering) apart, it remains largely unchanged from the edition that broke open the planetwide cultural silence about sexuality in 1972, and as such is still a kookily likeable, if frankly surreal, hybrid of two books in one: half tender beginners’ manual (advice on blowjob technique; encouragement to role-play), half extremely eccentric collection of exhausting-sounding “positions” sourced from “ancient Oriental texts”, all tirelessly test-driven by Comfort and his wife and demonstrated via pencilled illustrations with a positively doublejointed enthusiasm.

It is also, for all its randy explicitness, (still) characterised by a kind of Vaseline-lensed reliance on dainty metaphor (chapter titles include Appetisers, Main Courses, and Sauces And Pickles; oral sex is “mouth music”). This empire-mapping, Flavours-From-Round-The-World gourmand’s internationalism and diffident, almost courtly, male bias is all rather Edwardian: you imagine Comfort as a kind of benevolent, dotty Phileas Fogg, going round the world lovingly pinning all those local variations down like butterflies.

The 2003 edition features updated sections on monogamy, equality, hormone replacement therapy, Viagra and of course HIV - and new drawings, redone for the 1992 edition. His modified Will Oldham is gone; her hair, as well, is long rather than modishly nape-short: she’s a kind of proto-Rachel from Friends, or a slightly bed-mussed Nigella Lawson. Her underarm thatches, however, remain: so, too, is there no evidence of the Brazilian wax, or its more moderate north-European counterparts - which, had he lived to see them, Comfort probably would have considered vandalism on a par with throwing prawn mayonnaise on the Mona Lisa.

In any case, the original decision to use line drawings in 1972 was not an aesthetic choice but a necessity. “They really scratched their heads on that one,” Nicholas Comfort explains, “because they didn’t want to do anything that would lead to them being prosecuted for obscenity. But at the same time it was quite clearly pointless to produce a sex manual where you couldn’t see what people were doing. They thought of hiring prostitutes – you know, in those days, you had inflatable dummies illustrating stuff, it was ludicrous. And eventually they came to a complete stop, until the guy who was doing the artwork for the book said to the art director, ‘Why don’t I and my wife simply do the things in the book, someone photograph them, and then I’ll do line drawings from the photographs?’’ And that’s what they did. And they’re the bearded man and the lady. They’re a lovely couple, they’ve been married now for 35 years.”

To this reader’s eyes, the revision might have taken account of more than a few gauche haircuts, though. The book’s unmistakeable male bias, while so comical as to be inoffensive, nevertheless means you can’t take it seriously, surely, as a “sex” manual. Despite the repeated advice that “women are people, and equal” (er, thanks) there still exist slightly jarring suggestions like: “If the man needs her [to be] flat to finish, take her… turn her…” where a female reader begins to feel less like the co-author of a saucy sex-capade and more like a pancake, mid-recipe.

Then there’s homosexuality, which does not even rate a sub-heading. “I’m not sure my father… My father never…” Comfort pauses and begins again. “He would have defended to the death the right of anyone to be gay. But I think he (pause) … he regarded heterosexual sex as natural – or as most natural, if you like. And that’s what he wrote about.”

Despite these undoubted (and ultimately unignorable) flaws, the fact remains that the book’s cultural significance can’t be overestimated. “My father decided he needed to write the book in the 1940s, in fact,” says Comfort, “because he was very concerned about the ignorance he found as a doctor among members of the public about sex. He also felt that politicians, church leaders and doctors had a fair amount to answer for, in terms of instilling fear and ignorance into people as a means of control. He wanted to produce a liberating book.”

For that alone, more power to his… ahem, big toe.