"When does a story end?" Hugo Vickers wonders halfway through this mesmerisingly awful book about the last years of Wallis Simpson (pictured), adding "Perhaps it never does." For Vickers, who has been obsessed for decades with every detail of the Duchess of Windsor's life and long-drawn-out death, this is clearly true. For most people, however, the story of Edward VIII's abdication and its aftermath is fixed in a handful of phrases and images. "The woman I love", "impossible to discharge my duties as king as I would wish to do", with the striking emphasis on the "I", the rigidly posed wedding photograph showing Wallis with her snake hips and heavy, jolie-laide features all shadows and angles, the epitome of art deco elegance. After that come the long years of exile and speculation, his death, her haunted and much lifted face at the window of Buckingham Palace and then a slow fade, bed-ridden in the Bois de Boulogne, to her own end.

It is a story with a certain glamour and narrative rightness about it. The Windsors can be cast as the wrong but romantic counterpoint to their replacements, the dutiful but dowdy George VI and Queen Elizabeth. Or Wallis can be demonised as the femme fatale, the wicked American divorcee who "pinched our king". Neither of these variants will do for Vickers. He wants to get behind the image, feeling that his subject has been maligned, particularly by those previous biographers whom he characterises as "the intelligent and Establishment" writers. These people, he explains in aggrieved tones, "are well educated, they read serious books and some enjoy the arts in their highest form"; they are therefore, he believes, unable to understand a person who "did not involve herself in cultural activities".

In attempting to redress the balance Vickers has succeeded up to a point. Nobody could call this book intelligent. His method is to accumulate an immense amount of detail, mostly from the duchess's hostile staff, from which he selects such nuggets as that her nurses once requested single yoghurts for lunch and that in her last years she no longer had monogrammed sheets on the bed. Every dog she ever had is named and a great many brooches are listed. But on the bigger questions, he is vague, and of the Windsors' attitude to Nazi Germany at the time of the fall of France, it will not "suffice it to say" that they were "not disloyal".

Vickers himself looms large in the story despite never having met his subject. The closest he came was once to be in the room when her secretary took a phone call from her. For the rest of the time he was happy to hang out with the hangers-on and unblushingly records regular visits to peer over the garden wall which enable him to tell us that in 1985 "the lawn was very green". When the reader can see past her Pooterish champion, the picture of the duchess that emerges is not attractive. The best even Vickers can say of her is that she was stylish and elegant and, by his reckoning, descended from Henry III. The "friends" he quotes are not noticeably warm in their recollections. Cordelia Biddle, who had known Wallis since schooldays, remembered that "she was always a flirt . . . If she wanted something she went after it." Cecil Beaton later described her ambiguously as "intelligent within her vast limitations" and Vickers can do no more about her acknowledged tendency to manipulate the truth than to describe her variously as "a great actress" and "a typical Gemini".

The "untold story" of her last years is not tragic but sadly familiar: a frail old woman, exploited by those who were paid to care for her. Vickers's main impetus in this part of the book is a hatred of the duchess's lawyer, Suzanne Blum, with whom he seems almost as obsessed as with the duchess herself. Yet much of what he says is based on gossip. There is still less to support his unpleasant allegations against her long-serving butler, Georges, whom Vickers calls a "crook", "believed" to have taken a bribe and colluded in forgery.

It is hard not to think that Vickers's real grudge against Blum is jealousy. He cannot bear the fact that the frumpy lawyer with the ill-fitting wig was the duchess's close companion. After her health collapsed, the last 11 years of the duchess's life were dragged out in the midst of an accelerating danse macabre of acolytes, biographers and Blum jostling for the spoils and a seat at the funeral. For this Vickers was finally on the spot and so excited to be at the duchess's "last party" that he finds nothing tasteless about giving the guest list, the placement (with diagram), the price of the bouquet from Estée Lauder and the fact that the ex-wife of the Earl of Dudley was annoyed to see the present countess, "the former film star Maureen Swanson", wearing the Dudley pearls.

By the end the "poor little person"  as Diana Mitford called the duchess  far from being vindicated, is shown as stupid and venal, the moment of glamour in the 1930s just that, a chance whereby she caught the light of history. As for her biographer, our last sight of him is consistently bathetic as he leaves the sale of the duchess's effects, having successfully bid for a monogrammed bathmat.

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