If the word "gentry" conjures up any image today it is as likely to come from fiction as fact. Sir Walter Elliot, perhaps, in Persuasion, reassuring himself with his copy of the Baronetage by looking over the generations of Walters and Elizabeths from whom he is descended. Such dynastic confidence, like the art that celebrated it in the landscapes of Gainsborough and the great prodigy houses of the Elizabethans, was always something of an optical illusion, a pious hope of permanence that contrasted with a fragile reality as old families declined and new ones elbowed their way in.

Standards were already slipping by 1555 when the anonymous author of a treatise on the education of gentlemen complained at length about cash for honours and other "dark  practices" that allowed men without "lerning [sic] nor worthiness", to get their hands on landed estates. A hundred years later the long-established Oxinden family in Kent were sneering at their arriviste neighbour Basil Dixwell and his new house Broome Park, for which Dixwell had not only had to buy his own furniture but also "thousands" of his own bricks.

Yet the permeability of the gentry was in part what defined it. It might marry into the aristocracy but it had its feet in farming and the professions. By the time Jane Austen was writing, this flexibility had become a tenet of patriotic faith. On the one hand it allowed Mrs Bennet to entertain spectacular hopes for her daughters, and on the other it was responsible for saving the country from the horrors of the French revolution. This too was something of a myth, but it is true enough to make the idea of the gentry a slippery one and Adam Nicolson's determination to trace its continuity through the fortunes of a dozen families from 1410 to the present results in a curious, incoherent book, a mixture of telling detail and sentimental generalisations.

Among those from whom Elliot would not have cared to claim descent was Sir William Plumpton, who careered around Yorkshire in the 15th century with a band of armed tenants. Plumpton's response to a demand for taxes from the Cardinal Archbishop of York was to ride the archbishop's men off his land and, when they surrendered, to kill them in cold blood. It was a world of violence and corruption in which one bad decision could lead to disaster. The Plumptons' mistake was to back the wrong side in the wars of the roses and William's son was one of about 28,000 men killed on Palm Sunday 1461 at the battle of Towton. Towton, despite its scale and savagery, has never joined Hastings and Bosworth on the list of famous British battles  its site has only recently been excavated but Nicolson's account of the swath it cut through one family brings home its true enormity.

The book is studded with such compelling individual stories. There is Sir John Oglander, the otherwise obscure owner of an estate on the Isle of Wight. We watch him build carefully on his inheritance, improve his house and chastise himself for overspending on flowers for the garden. Suddenly his son dies of smallpox, the civil wars turn his world "tosi-turvey", and he takes to writing his journal in his own blood and tears. But Oglander's interest in himself and his feelings doesn't just mark him out as a different sort of person from Plumpton, it reveals him as the product of a quite different, post-Renaissance age and sensibility.

Oglander has more in common with Michel de Montaigne, in whose essays the literature of self-examination and profitable digression was born and launched to an enthusiastic readership on both sides of the Channel. Montaigne, too, was preoccupied with preserving his modest family estate in Aquitaine through difficult political times. He fits any definition of gentry. But he was not of course English and it is this dubious concept of Englishness, the glue with which Nicolson tries to stick the disparate parts of his book together, that in the end undermines it.

His England, an advertiser's dreamscape where "a lemon-yellow sun shines down hazily on Gloucestershire" while indoors flames are "glimmering on the polished oak", would have meant little to most of his subjects. The book's premise that "no country has described itself so intimately and for so long as this one" is questionable, but the English gentry certainly had a great deal to say and by Nicolson's own account in all their ruminations on houses, dogs, hunting and advantageous marriages they hardly ever, outside wartime, mentioned England. Of his dozen families two were fighting their fellow countrymen in civil wars while of the rest the Lascelles made their fortune in Barbados, the Pinckneys went to the United States and actually fought against Britain in the war of independence, the spendthrift Capels decamped to avoid their creditors and the Hugheses were Welsh.

A sense of place was no doubt deeply rooted in individuals, but so it was in France and Germany, where pride in national heritage and an inclination to preserve it was manifest long before any such moves were made in England. Widespread enthusiasm for the Englishness that Nicolson tries to read back into history belongs, like the mushrooming membership of the National Trust, The Buildings of England, the last night of the Proms and the green belt, almost entirely to the last half of the 20th century. Where Nicolson is probably right, however, is that the gentry can adapt to take advantage of it. It may now be more a matter of holiday lets in the orangery rather than broadswords, but there will be another fiction as persuasive as Gainsborough's to carry them on, for a while.

 Rosemary Hill's God's Architect: Pugin and the Building of Romantic Britain is published by Penguin.