The Devonshires: the story of a family and a nation, By Roy Hattersley

Derbyshire is the most beautiful county in England, and handsome Chatsworth the most picturesquely set of all great houses. The Cavendish Dukes of Devonshire, unconnected with the western county, owned lots of the northern one. The first Duke employed William Talman to produce a creditable property. Its essential form was accomplished for the fifth Duke by the neo-classical architect Jeffrey Wyattville.

With shrewd additions and good politics, the Cavendishes came to own an awful lot more, and to count extensively in government. They had risen across the late 16th century: Baron, then Earl, under James I; Duke from William III. Initial ascent came through Bess of Hardwicke - big house, big fortune, four husbands. Her Cavendish second husband begot a line to enjoy the land and income. They would stay reliably on the winning side, excepting an ill-advised loyalty to Charles I. But, always seriously Protestant, they rightly feared subordination to Louis XIV by the later Stuarts' wistful taste for absolutism and Holy Mother Church. Helping to get rid of James II, William, fourth Earl, chimed in with the good sense of English history.

He refined his commitment by plumping for winnable William of Orange. Within five years, dukedom and extensive employment followed. Creditably, when the 1697 Turnham Green Plot aimed to "Shoot King William on the road", Devonshire tried to save its chief plotter, Sir John Fenwick. Chatsworth now held serious Whigs: oligarchs certainly, but decent hands for politics. The Cavendishes got richer, but they took government seriously.

The Fourth Duke, briefly prime minister, has been better known to us since discovery in 1982 of his Memoranda written across three years from 1759. A fortnight after Wolfe's victory at Quebec, Devonshire records: "Mr Pitt –Disposed to quit unless Lord Temple has the Garter". He tries talking Pitt out of his sulk, then persuades an irked George II to pass out the bauble and re-start Cabinet government. Not a brilliant political figure, he showed sense in his short, useful life.

Reclusive Henry Cavendish - son of a younger son - was brilliant, with his arsenic analysis, identification of polar minimum temperature, first studies of the properties of major gases, joint discovery of nitrogen, essential work on electricity, and near-exact calculation of the earth's density. Compare and contrast: Amanda Foreman's account of the ubiquitously adulterous Georgiana, wife of the fifth Duke, sells on. Through the last decades of the 18th century, Georgiana's life was a running farce, flecked with febrile headlines. Georgiana, like Diana, was a Spencer; Cavendish women had more sense. She had love affairs, heterosexual and lesbian. She gambled compulsively. Variably in the hands of sharks and her victims, she divided the proceeds of one such advance into two, put a half on the Oaks and the other on the Derby, losing both.

Georgiana had opinions beyond her intelligence, exultant about the French Revolution before compassionating its victims. She wrote to her children in her own blood. All her life she was a sweet, delinquent child.

Against this kitsch, her husband, whose dullness Roy Hattersley stresses, was willing in the aftermath of the Irish Rising to call for conciliation. Not all Cavendish politics was so visionary. Gladstone's lieutenant during the Irish Home Rule crises was a Marquis of Hartington, later eighth Duke. Called "Harty-Tarty" for a dully inveterate extra-marital life, he was idle and ignorant. Assured in his resentful inertia, his signature to all initiatives was "Better not". Harty was against Irish Home Rule; indeed, against the Irish. There was occasion. His nephew, conscientious Lord Frederick, came to Dublin as Gladstone's Lord Lieutenant with land reform plans but was, hours later, stabbed to death. The Devonshires took their knocks. In 1944 Billy Hartington, heir to it all, lost a by-election, went back to war and was shot by a sniper. Andrew Cavendish, next in line, outfaced nightmare death duties and delighted everyone who met him - including the author and me. The Devonshires continue; and so does Roy Hattersley's brilliant run of acute, stylish history books.