The letter, read out by Frieda Hughes, reveals her father's feelings on portraying his relationship with Plath - Frieda's mother - in verse, and how he wished he had done so years earlier.
It said: "I think those letters do release the story that everything I have written since the early 1960s has been evading.
"It was a kind of desperation that I finally did publish them - I had always thought them unpublishably raw and unguarded, simply too vulnerable. But then I just could not endure being blocked any longer.
"How strange that we have to make these public declarations of our secrets. But we do. If only I had done the equivalent 30 years ago, I might have had a more fruitful career - certainly a freer psychological life.
"Even now, the sensation of inner liberation - a huge, sudden possibility of new inner experience. Quite strange".
Birthday Letters is dedicated to Frieda and her brother Nicholas; Frieda's artwork "Fire" appears on the jacket cover. In a remarkable hat-trick of prizes, Hughes' book about himself and Plath, who committed suicide some 35 years ago and became a feminist icon, has now won the Whitbread Book of the Year and the T S Eliot and Forward poetry prizes. It was the second successive year that his work has won the Whitbread prize.
All but two of the poems were about his life with Plath and the book was the last to be published before Hughes' death last year, aged 68, after a battle with cancer.
Unusually for a book of poetry, it topped the bestseller lists when it was published, shifting 150,000 hardback copies in Britain alone.
The chairman of the nine-strong panel of judges, the Hon Raymond Seitz, former US ambassador to Britain, admitted that the judges were conscious it was a posthumous award, that Hughes had won last year with Tales from Ovid, and that it might be seen as a sentimental favourite. But he said: "There is a variety and depth, and always a virtuosity of words. In the end we simply asked ourselves which single book would we most want to take to the top of a mountain."
Mr Seitz said it had been a close contest between Hughes' book and a novel, Leading the Cheers, by Justin Cartwright. Two other books were shortlisted, The Last King of Scotland by Giles Foden, and the biography, Georgiana, Duchess of Devonshire, by Amanda Foreman.
Also presented last night was the pounds 10,000 Whitbread Children's Book of the Year, which was won by David Almond's Skellig. The writer spent five years working on an adult novel which was rejected by 33 publishers, yet he wrote Skellig in just three months and it was immediately snapped up by Hodder Children's Books.
Mr Seitz said: "The book was virtually a consensus decision and many of the judges thought it good enough to enter for the main book of the year title."
The annual award was founded by the Whitbread brewery in 1971 to "encourage, promote and celebrate the best of contemporary writing".Reuse content