Eclipse, By Nicholas Clee

When Thomas Babington Macaulay wanted to say that Boswell's biography of Johnson was the only one that counted, he lighted on a handy catch-phrase: "Eclipse first, the rest nowhere." This memorable line was coined at the Epsom races on 3 May 1769, when one Dennis O'Kelly predicted how the next race would go. He meant something precise: that the horse Eclipse would pass the finishing post before the others had reached the distance marker, so they would get no position and the only horse placed would be Eclipse, the winner.

O'Kelly's confidence was natural: he was Eclipse's owner. It was also justified. Eclipse won this race, as he did many others, and went on to become the most successful breeding stallion of all time. His bloodline still dominates: through male offspring alone, he is ancestor to around 95 percent of all today's thoroughbreds. Eclipse's genes have lived on, twinkling through the centuries like the remains of an exploded star.

Nicholas Clee tells Eclipse's story together with those of a number of his descendants and associated humans. It is a centaur of a book: half-human, half-horse. But Clee knows how to tell a gripping story: he weaves the halves together into a well-written narrative of social change. He shows us how, in racing as in other aspects of life, corporate greed and commodification have steadily taken over from the gentlemanly pursuit of pleasure.

The commodity at the heart of the story is a substance worth more than oil or fine wines: horse semen. Owners of the top stallions today charge six-figure sums per pop, and expect their animals to manage 100 or more "coverings" per season. But a lot can go wrong, and only a few have what it takes. O'Kelly was among the first to spot this quality, and to see that this was where racing's real money lay.

An astute businessman as well as a gambler, he was a well-known figure at 18th-century races. His bear-like form was recognisable from afar, and he makes a vivid character here. He was mocked in England for his Irish brogue and scruffy dress, but his financial judgement was sharper than his suit, and he left his heirs a fortune, including profitable Eclipse shares.

O'Kelly and his mistress, the brothel madam Charlotte Hayes – one of Eclipse's later co-owners - dominate the human half of the story. There is a downside, for when they die, they take much of the narrative interest with them. By contrast, Eclipse's death in 1789 marks the beginning of an afterlife just as absorbing as the life.

It launches us on a tour through a series of his most noteworthy descendants, from Hambletonian, Whalebone and Pot8os (a horse whose name seems to have arrived via text-message from the 21st century) to modern stars such as Nijinsky, who exerted a spell on Clee himself as a young race-going enthusiast. The book ends with a visit to what purports to be Eclipse's skeleton, at the Royal Veterinary College. It has more claim to authenticity than other relics, which have included nine hooves – surely more than any horse needs, however fast - and enough tail-hairs to stuff a sofa.

Assuming it is genuine, the skeleton is interesting precisely because it is unexceptional. Neither very large nor very sleekly designed, it excels simply by showing a perfect balance of thoroughbred qualities.

What made Eclipse immortal was not just this balance, but his ability to transmit it undiluted through the generations. His breeding rivals did not finish "nowhere": each finds his place in the genetic map. But, as this fascinating book makes clear, Eclipse is definitely still first among equines.

Sarah Bakewell's latest book is 'The English Dane' (Vintage)