We are taught to look at art in the right, dutiful way. Art has a profound value, and it is our job to seek it out. But I must confess that I often find my sense of duty slipping. When I look at a picture of a horse, my mind doesn't focus on recession or chiaroscuro. I am, after all, a horseman: and naturally, the first thing that crosses my mind is whether or not I would like to ride that horse.

I visited the National Archaeological Museum in Athens when I was covering the Olympic Games of 2004 and I found myself looking at these ancient horses and wondering what sort of paces you would require of a horse before the invention of saddles and stirrups.

My sister Rachel is a real art histo-rian. She lectures at the National and the Tate; she has written books about the pre-Raphaelites and Gustav Klimt. And we found ourselves talking about horses in art at a family gathering. So we made wild plans to put together a book, she doing the chiaroscuro and me doing the fetlocks.

Her job was the art: mine was a relationship between two species that has endured across the millennia, in which time horses have been prized first for their meat, then as transport and power and prestige, finally as bringers of nothing more than joy.

I can't help but look at a horsey picture as a horseman. But then for centuries, neither could anyone who looked at them. Horses were a part of daily life: everyone could tell a good horse from a poor one, a decent rider from a bad. Horsiness is, for many people who look at pictures, a lost language. So I've tried to act as interpreter.

Take the famous equestrian portrait of Charles I by Anthony Van Dyck. It shows a very kingly sort of king, his grandeur and (quite literally) his stature set off by the majesty of his horse. But as a horseman, you notice the horse's ears, turned back in annoyance, and the eye slightly rolled back. You notice also the long-levered bit and the rowelled spurs.

It is clear that the horse has just received a sharp correction from spur or bit, and is not happy. Charles is painted as boss. It is abundantly clear that the horse is his servant, not his friend. He is the master. His subjects later cut off his head: his horse might have been pleased if someone merely cut off his hands.

Time and again, I was delighted with the sexiness of the horsey life. I liked in particular Joseph Wright's picture of Mr and Mrs Thomas Coltman: a woman riding side-saddle, her husband negligently resting an arm across her thigh. The side-saddle is an instrument of liberation, not repression. It allowed women to control a horse at all paces: no longer did they have to sit sideways on a docile animal. They could take their reins and their destiny in their own hands.

Mr and Mrs Coltman are pictured in the utterly exclusive intimacy of married life, as the master's horse is brought out for him. It seems likely that the couple have come to their morning exercise straight from a horizontal gallop.

I thought it only right that Quercus, the publishers, should buy each of the authors our picture of choice. My sister went for the Kandinsky sketch for the cover of the Blue Rider almanac: a dream horse, a rider with a superman cloak that takes the shape of a great bird, galloping through a colourscape of lemon and violet: not horse as means of transport but rider in a transported state.

I think I'll have the Leonardo. We went for one of his sketchbook studies of a rearing horse. There is an informality and a dynamism about sketches that finished and formal paintings lack. The rearing horse we chose for the book shows an animal apparently playing in a field, expressing his nature by means of movement, as horses always do. The image combines scientific observation with passionate involvement, and it seems to me to get right at the heart of horsiness. It tells us why we humans cannot live without our horses, even though we no longer need them.

The Horse: A Celebration of Horses in Art by Rachel Barnes and Simon Barnes
Quercus, £25; 224pp Buy the book here

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