Indeed, when compared with the summer's other big releases, The Da Vinci Code even seems a little innovative - no mean feat for a film directed by Ron Howard, who is to movies what white lines are to the middle of the road. There are three spongy blockbusters soaked in trequel (The Fast and the Furious: Tokyo Drift, X Men 3 and Mission Impossible 3), one sequel (Pirates of the Caribbean 2), and two warmed-up film franchises: Superman and The Omen. Even worse are pointless remakes of Miami Vice and The Poseidon Adventure.

Hollywood's ability to turn base fiction into cinematic gold is nothing new. James Bond, for instance, first reared his head between the sheets of Ian Fleming's paper-thin thrillers. Other literary alumni include Hannibal Lecter, Vito Corleone and Scarlett O'Hara. Less well known are Forrest Gump, John 'Die Hard' McClane, Joe 'Midnight Cowboy' Buck and Father Lankester 'The Exorcist' Merrin.

Novel-inspired movies account for 23 of the past 50 academy awards for best picture, a ratio repeated in the Internet Movie Database's top 100 films: 41 adaptations in all, including the entire top five if you count The Shawshank Redemption (a short story) alongside the two Godfathers, one Lord of the Rings, and a Schindler's List. One-third of the world's highest-grossing films were novels first (eight of the top 10 derive from works by J.R.R Tolkein or J.K. Rowling).

Of course, literary adaptations are no guarantee of success. Indeed the screen version of The Da Vinci Code's fosters both excitement and tension - after all, Hollywood may well think that 40 million readers can't be wrong, but those same millions are likely to compare every frame of the movie with their own idea of Brown's story. One film. Forty million readers. There's a good chance someone is going to be disappointed.

Casting has its part to play. Tom Hanks, Ian McKellen and Audrey Tautou are all bankable enough - but will they match the images already in readers' minds?

Even the best actors struggle as cinema's brow goes higher. Pulp is fine, and fiction's half-way house is just dandy, too. Merchant Ivory made millions out of E.M. Forster, and Anthony Minghella raised Michael Ondaatje's The English Patient to a level it didn't know it had. However, the intricate plots of Jane Austen have caused more than one director to wobble. The wobble becomes a stumble when filmmakers condense the convoluted clauses that become sentences that extend into paragraphs that wind over the pages that comprise the chapters that make up the books that Henry James wrote.

At least three brave souls have adapted Ulysses, James Joyce's novel to end all novels. Each is worthy enough (Milo O'Shea is a wonderful Leopold Bloom in Joseph Strick's 1967 version), but they're beaten from the start. And here is that start. 'Stately, plump Buck Mulligan came from the stairhead, bearing a bowl of lather on which a mirror and a razor lay crossed.'

How does an actor, both plump and stately, bear the bowl without giving the prosaic impression of merely carrying it? How does a director catch its symbolism, and hold Joyce's mirror up to art, Ireland and religion? And just how do you speak the opening line - 'Introibo ad altare Dei' - without coming over a bit Monty Python?

Cinema and literature have their respective strengths and limits, and make different demands on our imagination. This is true for trash and Irish geniuses alike.

As with starts, so with endings. In Jaws, Martin Brody (Roy Scheider) is in big trouble. Rifle slung over one shoulder, he clings to the mast of a sinking ship. What's worse, he wears glasses - whatever he has to shoot had better be pretty big. Unfortunately for him, it is. The shark is great and white, and needless to say, it's ready to rumble.

The rest is violence. Brody growls the one-liner, 'Smile, you sonofabitch', and Jaws obliges, exposing the scuba tank he has been chewing like the butt of a big cigar. Brody shoots, he scores. Boom! It's a glorious and thrilling climax, made perfect when Scheider and Richard Dreyfuss wisecrack their way to shore like Bing Crosby and Bob Hope in a Road movie.

And the scene as it was in Peter Benchley's novel?: 'The fish came closer ... Nothing happened. [Brody] opened his eyes. The fish ... had stopped. And then, as Brody watched, the steel-grey body began to recede downward into the gloom. It seemed to fall away, an apparition evanescing into darkness.'

Director Steven Spielberg thought Benchley's end 'a downer' and was happy to ignore the serious point it makes. Jaws, the novel, is a best-seller with ambition. If evanescing apparitions don't convince you of this, the titles Benchley rejected might. Leviathan Rising was one, A Stillness in the Deep another. Benchley's shark is an amoral force of nature, and what amoral nature can unleash, so amoral nature can reclaim at the death. This downbeat conclusion makes an upscale argument: the fish may be bad, but it's just hungry. It's Benchley's humans that are capable of evil: the corruption, selfishness and greed so rife in Amity. Spielberg wanted none of this moral clutter in his showdown. His shark is less a prototype for Ridley Scott's disinterested Alien than a throwback to the westerns of Lee van Cleef.

Benchley thought Spielberg had sacrificed plausibility for cheap thrills. 'If I've got them for two hours', Spielberg told him, 'the audience will believe whatever I do in the last three minutes.' Or to paraphrase William Goldman: truth and reality are all very well, but neither holds a candle to believability.

So, which is The Da Vinci Code? Fish or foul? Foul or fair? Now, we'll find out at last.