Why Super Thursday was a red-letter day: Two collections of private correspondence have amazed publishers by topping the book charts

Author Nina Stibbe has had an unexpected success with her book, Love, Nina

Many may have written off the personal letter as a dying art, but two surprise best-sellers are rekindling the nation's love affair with handwritten notes this Christmas.

Buyers are scrambling to get hold of the unlikely hits, which are outselling some of the season's biggest celebrity biographies.

Love, Nina: Despatches from Family Life and Letters of Note are ahead of a number of titles released on "Super Thursday", the day publishers release their biggest books, which are expected to shift thousands of copies over Christmas. They include books by Sharon Osbourne, Katie Price, Johnny Vegas and EastEnders star June Brown.

Love, Nina, a collection of letters sent by author Nina Stibbe to her sister, has captivated thousands of readers with its fascinating insights into the life of a nanny working for a family at the heart of London's literary society during the 1980s.

Nina writes home about her encounters with literary figures including Alan Bennett and Claire Tomalin. The Independent's Marianne Levy writes of "the thrill of discovery" she experienced reading the private notes.

Letters of Note, which started life as a blog in 2009, has topped the Amazon anthologies chart for its reproductions of letters from the great and the good – including the Queen, Mick Jagger, Dorothy Parker and Ernest Hemingway.

Letters of Note started life as a blog

Author Shaun Usher, who put the anthology together, describes it as a "legitimate form of snooping". "Letters offer a view of people that you don't normally see. They reveal unguarded moments, where you can pour your mind out. Biographies used to be based on letters, but we don't have that any more."

Although "people are communicating more than ever", through texts and tweets, Usher says, the death of the letter is "closing off our generation's traceable history".

Penguin said it had shipped up to 28,000 copies of Love, Nina and predicted they would sell as many as 14,000 before Christmas Day. And Letters of Note has shifted thousands of copies after Usher's blog became an internet sensation, garnering more than 1.5 million visits a week.

In a recent survey, half of young people aged 12 to 17 said they had never sent a personal letter to family or friends; many said they found traditional post too inconvenient, time-consuming and expensive.

Joanna Prior, Penguin's managing director, said the letters have a "unique appeal": "Quite often there are one or two books that slip under the radar and tend to do very well. It just goes to show how a genuinely good writer can cut through those celebrity autobiographies."

Commenting on the success of the book, Marianne Levy said: "Celebrity autobiographies are usually bought purely for the face on the cover, without much consideration as to how the text actually reads. Love, Nina proves that buyers and readers of books do care about the text itself," she said. "Crucially for the Christmas market, it's an exceedingly warm book."

Di Speirs, Radio 4's books editor, said she chose Love, Nina as Book of the Week because it was "the perfect Christmas listen". "At Christmas you don't want to hear a book about brain surgery, or a serious political memoir," she said. "This is a book which lifts people's spirits."