TWO new books on Leonardo da Vinci have covers that are almost identical. Both authors have pored over Leonardo's notebooks, and claim to take us inside the mind of the Renaissance giant. Yet one book, a surprisingly short one, paints Leonardo as a genius, whereas the other, a doorstep of a volume, presents him uncut, looking something of a fallen angel. Which Leonardo you choose depends on whether you prefer your heroes on or off their pedestal.

Martin Kemp, an eminent Oxford art historian and Leonardo scholar, has condensed what he calls Leonardo's “strange career” as an artist, engineer and musician into a series of key moments. Writing his book in the Tuscan villa that was once home to a smiling housewife named Lisa, thought to be the model for the most famous painting in the world, Mr Kemp warms to the ambience of the place before launching into the essential facts about the man. Yet after an auspicious beginning, the book reads like a gallery guide to Leonardo, and this may be because Mr Kemp is organising a Europe-wide exhibition of Leonardo in 2006, called the Universal Leonardo Project. His is a convenient handbook for the show or for any of the 24 paintings he attributes to the artist, and the book is also worth buying for Mr Kemp's handy timeline and illustrated list of Leonardos at the back. The prose, however, is more efficient than uplifting.

Charles Nicholl's long biography of the master is more gratifying to read, yet it ties itself in knots trying to follow every lead that Leonardo, his contemporaries and a legion of scholars have left behind. The author's goal is to show not the genius but rather the man, and he does his best to drag Leonardo down to earth. He begins with an anecdote about a note of Leonardo's jotted in the margins of an exposition on geometry: he is stopping work, the note explains, because his soup is getting cold. More details follow about Leonardo's animal-loving vegetarianism and about his inability to get a job done on time. A Freudian analysis of Leonardo's paintings of the Holy Family attempts to expose the artist's problems with father figures (Joseph is always absent, you see). And you learn more perhaps than you might wish about the homoerotic impulses in Leonardo's angels. That Leonardo, like Michelangelo and Botticelli, was homosexual is not news, but the lurid details of his love life may surprise some.

Such information would be more enlightening if it informed an analysis of how Leonardo became the great creative thinker we now consider him to be. Yet the only clues about the development of Leonardo's exceptional mind come from the fact that he was the illegitimate son of a notary; as such, he was not allowed to follow in the family business and was therefore spared a rigid education.

Instead, he was sent with his mother to live on a Tuscan farm. There, a deep love for nature was fostered, while his mind was able to develop, unfettered to an unusual degree. Leonardo was then apprenticed to a Florentine artist, Verrocchio, at a time when many artists were interested in the fashionable new techniques of perspective and oil painting.

According to both books, Leonardo's fascination with engineering came from watching the construction of Brunelleschi's vast dome over the cathedral in Florence. Yet other young artists who worked in Medici Florence had also turned their hands to everything, from painting to architecture and interior decoration. So what was special about Leonardo?

First, as Giorgio Vasari said in the 16th century, Leonardo had a “heretical” state of mind. So great was his curiosity about how things worked that he would believe only what had been proved empirically before his eyes. The clandestine dissections that got him in trouble with the pope flowed logically from a desire to learn how the body worked, the better to be able to render it in art.

Second, Leonardo was obsessed with birds, and claimed that his first memory was of being visited by a red kite in his cradle. Mr Nicholl constructs intricate theories about this, finding hidden bird patterns in the folds of the skirt of the “Madonna of the Rocks”, among other places.

Mr Nicholl's book brings the reader no closer to the nature of Leonardo's genius, though a better understanding of the man, warts and all, does emerge—a subtler appreciation of a man, for instance, who devised war machines for the violent Cesare Borgia yet also bought caged birds to set them free. Leonardo's love of birds and his desire to “conquer the resistance of the air” is the lasting and original detail of this book. Metaphorically, Leonardo flew, because he was for ever asking why.