When these two fresh examples of Leonardilia plopped on to my mat, my prediction was that I would value Martin Kemp's effort more than Charles Nicholl's. Kemp is Britain's Mr Leonardo, with 30 years of front-line engagement with the magus under his belt. Charles Nicholl is a newcomer to the territory, a biographer for hire who has previously tackled Christopher Marlowe and Arthur Rimbaud, and whom I therefore had down as a gadfly. I was wrong about both books.
Kemp's Leonardo is almost entirely useless, a vain and scratchy publication that recycles the writer's well-known takes on the artist in what purports to be a new order, but which ends up as a mess. It really is a most unnecessary product, neither a biography nor a thematic overview, but an awkward federation of essays on assorted Leonardo topics. There are chapters on Leonardo's anatomy, on his posthumous fame, on his fascination with the eye. A tiny and worthless gallery of black-and-white, thumbnail reproductions of his paintings has been tacked on to the end. The organising principle governing the work seems no more rigorous than chucking darts at a board.
In his introduction, Kemp tells us, with misguided enthusiasm, that he came up with a first draft of his book while incarcerated for ten days in the Tuscan villa once inhabited by Lisa Gherardini, the real Mona Lisa. It reads like it. Ten days' worth of pre-existing and familiar know-ledge have been spewed out and hastily rearranged. How dare this book be as sloppy as it is? We read at the off that Leonardo was born in Vinci at 3am on 15 April 1452, when in fact, as Nicholl carefully points out, it was actually three hours after vespers, which puts the time around 10.30pm. A small point, perhaps, but one indicative of Kemp's cavalier mood. On page 12, we learn that Leonardo's beloved servant Salai, to whom much was left after Leonardo's death, was killed in 1424. Yet Leonardo died in 1519. A typo, no doubt, but one that poisons your trust.
While Martin Kemp's enthusiasm for Leonardo feels dulled by overfamiliarity, Charles Nicholl immediately makes clear his status as a Leonardo virgin. He is the first writer in living memory to take seriously Leonardo's dodgy pre-Freudian memory of being attacked by a large bird while lying in his cot. Freud believed the bird to be a vulture, and based a florid psychosexual reading of Leonardo on this weird infant memory. A silly tradition soon grew up devoted to spotting secret vulture shapes in the paintings. It later transpired that "vulture" was a mistranslation, and that Leonardo had actually been remembering a kite.
Nicholl takes everything seriously - every snippet, every tradition, every speculation - examining and re-examining Leonardo's life in a prodigious display of scholarship. His aim is obviously to provide the definitive modern biography: not an art history book as such, but a resonant uber-life in which not just Leonardo comes to life, but also his contemporaries, his times and his places.
It took me a while to get used to this approach. So little is known about the young Leonardo that other writers have devoted five pages to what Nicholl does in a hundred. Once you get the hang of Nicholl's shaggy-dog rhythms, however, the book begins to grow in stature as well as length. Being a Leonardo virgin keeps the author alert and open. He is not afraid to speculate about Leonardo's sexuality, or about anything else. You soon learn to pick your own path through the spoil-heaps of information and, after a while, a distinctly fresh Leonardo begins to emerge.
One of Nicholl's amateur art-sleuth weaknesses is a tendency to spot self- portraits all over the shop. Unlike Kemp, Nicholl believes the famous red-chalk image of the old man with the big white beard is an accurate representation of Leonardo. Whether it is him or not - and I'm with Kemp on this one - the image of the bearded god-figure has had an enormous influence on our view of the magus. The single best thing Nicholl does in his ultimately superb biography is to make Leonardo younger: to present us with a beardless, snobbish, fastidious, effeminate dandy, interestingly cassata'd with darknesses. This stubborn display of research and speculation prods into the spotlight a proper, flawed human being, in place of the quasi-biblical bearded genius who usually confronts us.
The chapters on Leonardo's 18-year sojourn in Milan, during which he painted The Last Supper and not much else (as was his tendency), are particularly meaty. His local protector Ludovico Sforza - called Il Moro ("the Moor") because of his saturnine nature - steps out of the wings. Ugh! What a grotesque man. The fashion in modern biography is to pack the work with exciting mini-biographies, and this one fleshes out Leonardo's Milanese circle excellently.
Even livelier is the tracing of Leonardo's disgraceful interlude with Cesare Borgia, when he tagged along with the murderous and violent duke in the conquest of Romagna. Leonardo was Borgia's in-house war guru, charged with inventing killing techniques. He had, as Kemp also makes clear, a most odd career. Almost all of it was spent as a lickspittle of the rich and powerful, a trophy genius to parade before visitors, with his on-tap brilliance, his crackerjack mind, his surprising fund of salty jokes, and that unparalleled talent for visualising crackpot ideas that the rest of us wake up and forget.
No one will ever write a definitive biography of Leonardo; there will always be too much space between the facts. But I cannot see anyone improving significantly on Charles Nicholl's effort.
Waldemar Januszczak is art critic of the Sunday Times