Sheila Murphy, of Hanff's publishers Aurum Press, said the author died on Wednesday afternoon in New York City.
Hanff captured the hearts of thousands by charting her correspondence from New York with an antiquarian bookseller in London between 1949 and 1969. The letters between the two ceased when bookseller, Frank Doel, died.
The success of the book, first published in 1971, led to its adaptation as a television drama, a West End play, and then a film starring Anne Bancroft and Anthony Hopkins. It has also been broadcast as a radio play.
In 1992, Hanff completed Letter from New York - an anthology of her monthly broadcasts for the BBC Radio 4 programme Woman's Hour in the 1980s.
Throughout her life, she was fascinated by literary London. In 1974, she wrote The Duchess of Bloomsbury Street which described how it felt to visit the city for the first time.
Hanff had arrived in London to publish 84 Charing Cross Road six months after the bookshop of which she wrote closed in 1971.
Her two other books, Underfoot in Showbusiness and Apple of My Eye look at life in New York.
Ms Murphy said: "Helen was one of the few writers I know who was dearly loved by booksellers.
"84 Charing Cross Road was a great example of the art of snappy letter writing. It's a classic that has never been out of print in 25 years. It's difficult to find classics that have been in print for 10 years.
"As long as books are read, people will recommend 84 Charing Cross Road to one another.
"When Helene was last in London five years ago she was on her third generation of fans. Readers had had the book recommended to them by their mothers."
The US-born author lived in Manhattan for most of her life. She had an apartment on East 72nd Street where fans continued to write to her up until her death. She never married and was childless.
She would have been 81 next week.