I picture the 10th (and last) Earl Fitzwilliam most vividly at a number of near-identical Conservative Party annual summer fetes at Milton Hall, near Peterborough.

He was all aristocratic affability as he strode among the stalls on his front 'lawn'; later his features took on a more businesslike demeanour as he mounted the podium for the rallying call to the Tory troops that traditionally ended the day.

By now, he was surrounded by smart-suited spivvy sycophants, none smarter suited, spivvier or more sycophantic than the brilliantined local MP Sir Harmar Nicholls, a one-time President of the Wallpaper and Paint Retailers' Association. 'Smarmy', as he was known, was dad to the actress later to achieve fame as Audrey Roberts in Coronation Street. Ain't life strange!

The Milton Fete always featured a stall run by the earl's wife and stocked with some of her own lovely cast-off clothes. The 'Countess's Boutique' gave the opportunity to people who would otherwise never be able to afford them to snap up designer garments - A Chanel suit perhaps, an Yves St Laurent dress, a Balmain skirt - for a fraction of their original cost. Whether any of them was ever worn by the red-faced Tory ladies of the Fens, I rather doubt; for Lady Fitzwilliam clearly lived by the precept that one could never be too thin - or too rich.

Of Lord Fitzwilliam's very considerable fortune, it was always said - by those who claimed to be 'in the know' - that it had been gained by methods not entirely fair; in the words of Banquo that he had played "most foully for't". It was darkly hinted that he had achieved his position in life by calling his older brother a bastard, and thereby wresting an inheritance that would otherwise not have been his. This, it turns out, is precisely what had happened; but since it had been at a court case in the year of my birth (1951), the matter was rather ancient history by the time I was to hear of it 20 years later.

Now I am interested to have chapter and verse of the affair at last, in one of the many strands of 'plot' involved in Catherine Bailey's excellent book Black Diamonds (Penguin/Viking, £20). Since reading it in the middle of last month, I have been singing its praises to anyone around to listen, ignoring the somewhat glazed expression that can appear on any friend's face when one's latest literary enthusiasm is being advertised. "What's it about?" people have asked. "About the best book from a first-time writer that I've read in years," I have replied, more than once.

Seriously, what it is about is hard to summarise, since it is about rather a lot. How much, you will have to find out for yourselves, for I have too little space to attempt a proper precis. Ms Bailey (above), who read history at Oxford, has subtitled her book The Rise and Fall of an English Dynasty, and this certainly covers a major part of the book's content. But there is rather a lot, too, about coalmining on which the Fitzwilliams' massive fortune was based, about politics, about social deprivation, about family feuds, and about gossipy activity involving the always rich and often famous. (In this category comes the affair between the Fitzwilliam heir Peter Milton and Jack Kennedy's married sister Kathleen 'Kick ' Hartington, who were killed in an air crash on their way to a dirty weekend together.) In the background throughout is the family's great Yorkshire mansion of Wentworth Woodhouse (Milton was but a satellite), the largest private house in Europe, where so much of the story is set.

It was here, in July 1972, that the aforementioned 10th earl - always known to his family as Tom - ordered his employees to burn the bulk of the estate's 20th-century records. Ms Bailey writes: "The documents were burned in a bonfire that blazed, night and day, for three weeks. Other smaller fires had preceded it: in a deliberate attempt to hide from history, the private papers of the 7th, 8th and 9th earls - the tenants of Wentworth in the first half of the 20th century - were destroyed."

You will note that the author refers here, as she does throughout the book, to 'Wentworth'. She justifies this by saying this is its local name. It seems a rum practice to me, though, given that this jewel of 18th-century architecture has a national reputation under its full name. With a room for every day of the year, the property was a nightmare for visitors unfamiliar with its layout. The answer was to distribute different coloured confetti for guests to scatter on their way to dinner, so they could follow the trail back to their rooms afterwards. Amazingly, one reclusive old gentleman, who bought the dilapidated house for a song, now lives there entirely alone and unknown to people in the neighbourhood.

Have I whetted your appetite yet . . .?