Intimate, revealing and previously unpublished, the Mitford letters present a vivid portrait of the six sisters and the age they lived in. Deborah, Dowager Duchess of Devonshire and the sole surviving Mitford, talks to John Preston about some of their more surprising revelations.

In June 1937, Deborah Mitford visited Munich where she took tea with Adolf Hitler. 'I went with my sister Unity and our mother to his flat,' she recalls. 'The atmosphere was rather awkward because neither my mother nor I could speak German. Unity and Hitler talked away while we sat there not being able to understand what they were saying.

'But one or two odd things happened at that tea. My mother and I were quite dirty because we'd come in an open car from Austria, so we went to wash in his bathroom. Hitler's towels, I remember, had "A.H." embroidered on them. I've always thought that was so strange, so unlike what you would expect of someone like that. I remember, too, that he rang a bell several times and no one came. That was a bit odd, too.'

Yet what seems oddest of all in retrospect is how little impression the occasion made on her. In a letter to her sister, Jessica, Deborah barely mentioned it, being far more interested in describing a café violinist whom she had fallen for. 'Well, I've never been very interested in politics, you see,' she says, laughing. 'And the truth is that I didn't give it much thought. If you sat in a room with Churchill you were aware of this tremendous charisma. Kennedy had it too. But Hitler didn't - not to me anyway.'

Deborah Mitford is the last of the Mitford sisters. All her five siblings have died, as has her husband, Andrew, the former Duke of Devonshire. But at 87, Deborah hums with vivacity, a slim, immaculately coiffed woman with bright blue eyes. You don't have to look hard to see why she was hailed as a famous beauty. Today she is wearing a blue shirt with a white collar. 'It's my district nurse look,' she says, an effect offset by a jewelled spider pinned to her left-hand collar which looks as if it might crawl up her throat at any moment.

She is sitting in a studio in the garden of the former vicarage she moved into when she left nearby Chatsworth 18 months ago. All around her, on steel shelves stretching from floor to ceiling, are files containing letters the Mitford sisters wrote to one another. A selection of these letters, edited by Diana Mitford's daughter-in-law Charlotte Mosley, is just about to be published. It's a book of more than 800 pages, but it represents just five per cent of the letters the sisters exchanged. They wrote to one another almost every day, and sometimes more than once, a correspondence that lasted from the 1920s until 2003, when Diana died.