As many kids have done before me, I lapped up Charlie and the Chocolate Factory when primary-school age. However, when I tried to tune in to the evening ITV show Roald Dahl's Tales of the Unexpected, I was forcibly ejected from the sitting room by my parents. The 70s title sequence of curvy female silhouettes dirty-dancing against a red background was enough to signal to them that this smut was not for me.

Thirty odd years on, the "national treasure" status of this complex character best known for his prolific children's fiction is now assured. It may not yet be printed in your average week-at-a-view desk diary but 13 September is Roald Dahl Day.

I've recently re-familiarised myself with Dahl's junior output through my eight-year-old. The story CDs recorded in his own seductively velvety tones are a handy kids' bedtime device for their soporific quality. Now his often overlooked adult content – the thought of which so horrified my folks – is also being reissued as audiobooks, fittingly read by a brace of national treasures.

Even if his best-known books date from the 1960s onwards, Dahl can still command headlines. When his supermodel granddaughter Sophie appeared on Radio 4's Today programme last year, the public were outraged that she seemed to be asking for public funding for restoration of his writing hut. The issue has since been resolved, and as part of Roald Dahl day the interior of the outbuilding has been recreated at the Roald Dahl Museum in his hometown of Great Missenden, Buckinghamshire, funded by the family and his charitable foundation. The saga's twists and turns were meticulously covered by specialist site Shedworking.

This controversy is absent from Michael Rosen's new biography – perhaps unsurprisingly given that it is published by children's imprint Puffin Books. Rosen includes new archival material – school photos and letters to Mama from his war hero and boarding school days. It's a warm portrait that largely concentrates on his young life and doesn't dwell on episodes like the breakdown of his first marriage because, as his publicist points out, it's primarily for 8- to 10-year-olds. Adult biographer Jeremy Treglown meanwhile labels Dahl "a fantasist, an anti-semite, a bully and a self publicising trouble-maker" and describes his extensive sexual liaisons when working in the US.

It's the books themselves that fascinate most though: his adult material is not for prudes. The "horny" character Uncle Oswald, "the greatest fornicator of all time", gets a novel to himself and pops up in Switch Bitch – a collection of short stories that originally appeared in Playboy. In Bitch, following a scientific experiment to produce a scent that stimulates sexual attraction, he turns into a seven-feet penis that floats into space, declaring "But tell me truly, did you ever see / a sexual organ quite so grand as me?".

There are running themes of vice and eroticism and as well as the darker cruel exploits and moralistic comeuppence that are familiar from his kids novels. The Great Switcheroo is a tale of wife swapping for a one-off darkened sex session between neighbouring couples without the wives knowing. Chocolate manufacturing or oversized peaches this ain't, but then again these stories were designed for an adolescent male audience in a pre-politically correct age where casual misogyny went more unnoticed than now.

Hagiography is usually reserved for those who have departed us. Roald Dahl Day festivities are taking place at various venues across two countries. The Norwegian Church Arts Centre in Cardiff, where Dahl was christened, has a day of activities. There is also a Roald Dahl gallery of the nearby Aylesbury County Museum, which recently ran a Roald Dahl festival..

His work may not be as zeitgeisty as EL James's Fifty Shades trilogy, but with amassed UK sales upwards of 37m books and more than a million of them shifted every year, Dahl still sells by the shed-load. Nonetheless, in the runup to Roald Dahl day some 22 years after his death, perhaps the two authors are not as far removed from each other as one might initially think.