And as for Dan Brown's book, the 80 million copies it has sold should be a killer argument that it's a great story. Should be, but isn't. To say that the plot of The Da Vinci Code is a load of old cod would be unkind to that useful fish.
But one good thing can be said: nonsensical and all as it is, the book introduced a great many people to the mystery of Leonardo da Vinci. And, though it only appears in the film for a few seconds, the book renewed interest in the world's most famous painting, the Mona Lisa.
In her first novel, British writer and academic Lucille Turner has had the bright idea of explaining Da Vinci through Lisa Gherardini, the subject of the painting. As a solitary child, one of Leonardo's few friends was Lisa, a girl who used to spy on him in his workshop.
Born in 1452, one of 17 children of a legal clerk, Leonardo was a Renaissance genius, one of the most remarkable men who ever lived. But he was a restless spirit: he finished very few paintings in his lifetime and much of his energy seems to have gone into his notebooks -- Microsoft's Bill Gates spent $30m buying one of them, the so-called Leicester Codex.
Whether the Mona Lisa is a finished work is debatable. Leonardo didn't think so: he failed to deliver the portrait to the man who commissioned it, Francesco del Giocondo, who married Lisa when she was only 15.
Although Lisa had six children and was apparently a good and faithful spouse, it's not her virtue or even her beauty that has made her in some ways the most talked about woman in history.
The question asked in the song Nat King Cole sings in Neil Jordan's 1987 film suggests an explanation: "Is it only cause you're lonely/They have blamed you/For that Mona Lisa strangeness in your smile?"
In Jordan's film the heroine is a prostitute. Certainly, sex comes into the way she is portrayed by Da Vinci -- but with equal certainty we can say that Leonardo didn't fancy Lisa sexually. After all, when he was a young man, he was arrested three times for sodomy. And he had a boy servant with whom his relations were far from chaste.
Turner has not got much time for the homoerotic. Her Leonardo is only hot for art. He's finicky, almost obsessively neat and tidy, a vegetarian, a near anorexic.
In other words there is something feminine in Turner's depiction of the character -- and in fact art historians have often speculated that the Mona Lisa is actually a man.
But there's nothing female or male about the way Turner portrays Da Vinci's devotion to his work. It could be said she makes him out to be a monomaniac, but monomaniacs are usually bores with only one subject in mind, usually themselves, whereas Leonardo was crazed by multiple obsessions.
Really, he was a mad inventor, a sculptor, an engineer, a geologist, a musician, a scientist, a builder of flying machines, a pioneer in the study of optics, a man who cut up dead bodies and drew the corpses -- at one point in this book he boils a human eye to preserve it.
He was also something of a diplomat. In 1482, Lorenzo di Medici sent him from Florence to make peace with the Duke of Milan, Ludovico Sforza.
In the event, Da Vinci stayed in Milan for 17 years. Turner writes a telling description of Leonardo making a model for a 75-tonne bronze statue of Sforza, the first time such a massive smelting operation had ever been attempted. In the event, Sforza preferred to use the bronze to make cannon guns.
Turner is also interesting about Da Vinci's relationships with the philosopher Machiavelli and the painter Botticelli.
But, though the book is a decent length, 293 pages, it hardly scratches the surface of an incredibly busy and complex life. Even so, it's infinitely more interesting than either Dan Brown's The Da Vinci Code or the film based on it.
And the Mona Lisa? Turner explains the mystery by suggesting that Leonardo could never forget that Lisa witnessed one of his earliest and most gruesome experiments: a monster he constructed from the dead bodies of a dog, a squirrel, a lizard and a cockerel. It's an entertainingly weird tale -- like Turner's novel.
Brian Lynch is a poet, novelist and screenwriter