Forget Ana Steele and Christian Grey – the relationship between books and the internet gets weirder and more intense all the time. Yesterday we heard that the book in which these characters first appear – Fifty Shades of Grey – had become the fastest adult paperback novel to hit a million sales.

It smashed the record set by the equally admirable The Da Vinci Code, and set a further record for weekly paperback sales (379,889). But what's weird about E L James's saga of S&M and soft porn isn't the shifting of units. It's the madness of the fans.

Hard on the heels of the book's success came a number of "fan-fiction" spin-offs, in which readers wrote their own alternative versions of the plot, using the characters of Ana and Christian, changing their ages, switching their roles and having them embark on more athletic feats of colonic gratification than their original ever dreamed of.

These shocking, if harmless, acts of appropriation weren't unprecedented. The website Fanfiction.com had played host to scores of retelling of the Harry Potter books, in which you could find Ron Weasley being buggered by Draco Malfoy and Hermione developing a lesbian crush on Dolores Umbridge. The Twilight books by Stephenie Meyer met the same fate – and by a pleasing coincidence, E L James was one of the deranged rewriters. She changed the names of the characters from Edward and Bella to Christian and Anastasia, called her sexed-up version of their affair Master of the Universe and published it as Fifty Shades.

It wouldn't do to be sniffy about works of fiction that feed off others. Literary history is full of important works that are shameless retellings of others. Don Quixote was essentially a piss-take of courtly love stories. Henry Fielding began his career with Shamela, a gleefully immoral version of Richardson's virtuous Pamela. More recently, the divine Zadie Smith's On Beauty was a recasting of Forster's Howards End.

But we might begin to wonder what has become of readers. Once they were content to read a book, enjoy the plot and feel warmth or dislike for the characters. Now, increasing numbers are driven to invade the book's pages, wrestle the characters away from their creator and provide their own plot twists. Most, of course, are wholly unequipped to write their own novel or to handle prose with much grace or vividness. But what happens if, in the free-for-all of online publication, a spin-off book starts to eclipse the original from which it derives? Does the author bring in the lawyers? Or will it be too late to stop the bastard offspring from zooming all over cyberspace?

And what do we make of the success of Unbound, the "crowd-funded" publisher which celebrates its first birthday this month? It has launched 14 books, not by hopeless internet self-publishers, but by serious writers, among them Kate Mosse and the former Python Terry Jones. The idea is that authors pitch their ideas for as yet unwritten books to the passing trade who can subscribe sums of money towards the book's publication, until there's enough to make the writer start writing and to pay for hardback print run.

The company has 20,000 subscribers and 70 projects under way. What alarms me, though, is the idea of a tariff in which the reader gets ever closer to the author – £150 buys you an invite to his launch party; paying £250 means you can meet him for lunch. For £500 you can probably look though his shed window. For £750 you can hang out inside the shed with a bottle of burgundy. For £1,000 you can turn up in the plot and shag the heroine in Chapter 9…

OK I'm inventing some of these. But the day is coming (I seem to have turned into Cassandra) when novels won't be written from the singular imagination of the author, but by the collective imagination of the writerly wannabes who'll pay for its  existence.

Munch will set you screaming

The new Edvard Munch exhibition at Tate Modern isn't exactly a first-date treat. By the time you've stumbled through 10 rooms of family deaths, weeping, surgery, claustrophobia, cold Norwegian skies and faces in alarming close-up in the corner of landscapes, you'll need a stiff slug of Akkevit.

What drives you slightly nuts is its emotional inconsistency. Munch painted the same scenes over and over again, years apart. And just when you've gazed at the wretchedly tragic woman in Ashes (1895), who's clearly been abused at a picnic by the remorseful man hiding his face, you turn and discover the same scene, painted 30 years later, in which the woman looks triumphant and vampiric, and the poor man is the victim.

Honestly, that Munch. He can't even be consistently aghast. There's probably a version of The Scream somewhere displaying a horrible grin...