The following correction was printed in the Guardian's Corrections and clarifications column, Saturday 11 July 2009

Below we mentioned a 1976 novel, Star Wars: From the Adventures of Luke Skywalker, as an example of a book written by a film's director. In this case, the novel was ghostwritten by Alan Dean Foster.

Authors of film novelisations, not unlike pornographers, rarely get the respect they deserve. Generally, practitioners of such genres are held in contempt by writers and critics, who dismiss them as hacks.

That is not entirely fair. After all, most books by movie stars, politicians, athletes and businessmen are cobbled together by ghostwriters, furnishing the alleged "author" with an eloquence and turn of phrase that would otherwise not be available to them. Yet reviewers will routinely praise manufactured books - withholding their criticism of the ghostwriters who participate in such literary fraud - while looking down their noses at those who toil in the novelising fields.

Such selectivity defies logic. Novelisations, at the very least, are the work of the authors whose names appear on the cover. Well, more or less. Technically, they are the rewording of screenplays written by other people, supplemented by vivid descriptions of images furnished by the directors. But sometimes, as in the case of the 1976 release Star Wars: From the Adventures of Luke Skywalker, they are written by the directors themselves. (Tellingly, Han Solo does not appear on the cover of the original Star Wars novelisation, nor is he mentioned in the jacket copy, indicating that Mark Hamill, and not Harrison Ford, was the one originally destined for stardom.) So novelisations are not what anyone would call "original". But no one pretends they are.

One major reason novelisers are scorned by the writing community is that the genealogy of the works in this genre is so complex that it is not always clear what relationship to a film a novelisation bears. Another is the fact that novelisers make so much money. Bear in mind that novelisations have been around in one form or another almost since the birth of the movies. Sometimes their function is to drum up interest in a film that has already been shot but not yet released; sometimes to keep the public's interest stoked during that dark period between the end of a film's theatrical run and the moment it is released on DVD.

Novelisations, so it is rumoured, often contain supplementary material that make it easier to understand the film on which it is based. For example, the whole time I was watching Underworld: Rise of the Lycans, the third instalment in the Underworld series, I had a hard time figuring out why Lycans could sometimes get along quite nicely with werewolves, but at other times wanted to rip out their lungs and eat them. I was also confused as to why Lucian the Lycan could occasionally turn into a gigantic werewolf as if on cue, but other times had to lie there snivelling like a whipped cur while thrill-seeking lycanthropic flagellants shredded his naked flesh. Not until I read Underworld: Rise of the Lycans - The Novelisation, by Greg Cox, did it all became clear.

And not a moment too soon.

Similarly, the whole time I was watching Hannah Montana: The Movie, I did not realize that Miley Cyrus, Miley Stewart and Hannah Montana were all one person. The wig confused me. Originally, I thought Miley Stewart was the young Minnie Driver, while her alter ego was Miley Cyrus dressed up to look like Hillary Duff. It was simply too hard to follow; it was more confusing than The Matrix Revolutions. Not until I read Hannah Montana: The Junior Novel by NB Grace did I realise that Miley Stewart/Cyrus was not only Billy Ray Cyrus's daughter but Hannah Montana in disguise.

It is not always easy to figure out what is going on in the world of novelisations. Consider Terminator Salvation: The Official Movie Novelisation by Alan Dean Foster. Terminator Salvation: The Official Movie Novelisation is not to be confused with Timothy Zahn's Terminator Salvation: From the Ashes - The Official Movie Prequel. Nor is it to be confused with Terminator Salvation: Sand in the Gears - The Official Movie Prequel Graphic Novel.

Here, a bit of supplementary material about all this supplementary material may be helpful. Novelisations are based upon movies that already exist. Official prequels are novels based on the outline of a movie that has already been greenlighted, but may not yet have been shot. Prequels may thus contain scenes that ultimately get cut out of the finished film. For example, even if Hannah Montana ran away to join the Ladies' Taliban in the prequel to her next movie it wouldn't necessarily mean that she would do so in the upcoming film. In fact, it's pretty unlikely. It could simply be the mad, zany fantasy of some out-of-control prequelist.

In the world of novelisation there is also a subgenre called Unofficial Prequels, and very possibly Prequels to Sequels, Sequels to Prequels, Junior Prequels to Sequels, and Semi-Official Penultimate Junior Prequels to Ultimate Graphic Sequels. Presumably, if someone went out and wrote the prequel to a movie that was then abandoned, a first edition of the prequel would be worth an immense amount of money as a collectible to those who care about these sorts of things. It is even possible to imagine an unscrupulous producer buying up all the prequels to a project like, say, Watchmen IV: This Time the Film Won't Suck, and then sabotaging the project in order to goose the value of the prequels. Obviously, if Watchmen IV did get made and didn't end up sucking, the prequels would be worth nothing. On the other hand, the chances of Watchmen IV: This Time the Film Won't Suck not sucking are microscopic, especially if Zack Snyder directs it.

Novelisations are usually much easier to follow than the films they novelise. This is particularly true in the case of an incredibly sophisticated film such as Transformers: Revenge of the Fallen, a film of such byzantine complexity no ordinary teen mind could keep track of all that is transpiring within. One simply must have the Junior Novel to keep score.

Similarly, Terminator Salvation: The Official Movie Novelisation is easier to follow, and considerably less moronic, than the film upon which it is based. Alan Dean Foster writes in a direct, descriptive style, using un-Schwarzeneggerian words like "concise" and "disparity". The book is intelligent and lucid. The book makes sense. Unlike the movie, the book is not stupid.

Every once in a while, the author even gets a bit arty: "Elation replaced disquiet when he came across the codes for the perimeter defenses." Or: "The briefest of flickers of moonlight on cornea." Or this memorable exchange: "The younger man's shrug seemed to suggest that geographical designations like 'away' no longer held much in the way of relevance."

I am not going to argue that this is good writing. Not really good writing. Not really, really good writing. All right, it's bad writing. But at least Foster is making an effort. At least he's earning his paycheque. He has not simply gone on autopilot like the director of Terminator Salvation. And in Star Trek, his novelisation of the new movie of that name, his writing is even better. Foster is thus a noveliser who can rise to the occasion. Or sink to it, if required.

A very different case is Night at the Museum: Battle of the Smithsonian: A Junior Novelisation by Michael Anthony Steele. Unlike Night at the Museum: A Junior Novelisation, Leslie Goldman's rewrite of Milan Trenc's The Night at the Museum, (the storybook on which the film Night at the Museum is based), Night at the Museum: Battle of the Smithsonian: A Junior Novelisation is the junior novelisation of the sequel to the original Night at the Museum. It is thus a sort of a sequel to a film, and a sequel to the novelisation of a film, which was in itself the junior novelisation of the screenplay to a film which was based upon a book which was mostly pictures. Is that clear?

None of this has anything to do with the adult novelisation of the film, which in this case does not exist anyway. Battle of the Smithsonian: The Junior Novelisation is professionally crafted, and religiously faithful to the spirit of the movie. The author makes no attempt to stretch his wings artistically, hewing close to both the spirit and the dialogue of the film upon which the Junior Novelisation is based. Battle of the Smithsonian, like the film, relies on lines like: "Stop blinking and take me to the Cube of Rubik." This is known in the trade as Humour the Whole Family Can Enjoy. Well, some families.

The Underworld, Night at the Museum, Transformers, Terminator and Star Trek franchises are all films that depend heavily on special effects. Yet unlike the other books, the Hannah Montana novelisation has a phoned-in quality and falls completely flat. This, I suspect, is because Miley Cyrus is a bona fide superstar, with irreplaceable charisma, something that cannot be said of Christian Bale, Michael Sheen, Ben Stiller or Shia LaBoeuf, all of whom appear in films where they are more or less upstaged by the ordnance.

It is a telling statement that a noveliser can effortlessly write a readable book based on a motion picture filled with computer-generated monsters and get away with it, but has trouble when asked to bring a bubbly teen multiple-personality to life on the printed page. Perhaps there is still a place for human beings in films after all.