“Puffy and spinsterish,” she quipped of Miss Marple, her other famous sleuth. “The old spinster lady living in a village.”

Uttered in the reedy voice of Christie herself, these withering descriptions are contained on a cache of audiotapes, recently discovered in a dusty cardboard box in one of her former houses by her only grandson, Mathew Prichard.

The tapes - 27 reels running a total of more than 13 hours - are filled with Christie's painstaking dictation of her life story, rough material recorded in the early 1960s that eventually made up her autobiography, published posthumously in 1977. It stands as one of only a handful of recordings of Christie, the British mystery writer, who rarely agreed to be interviewed.

Christie's estate is expected to announce its discovery on Monday, the 118th anniversary of her birth, calling the tapes a rare find and a significant addition to the collection of memorabilia related to Christie.

In Britain the appetite for all things Agatha Christie is still fierce. Devoted fans still mark her birthday with a weeklong festival of theater performances, treasure hunts, teas and murder-mystery parties. And while her books have never been considered high literary art, more than 500,000 copies of them are sold in Britain each year. She has been outsold in volume only by Shakespeare and the Bible.

Taking into account such strong interest, Christie's estate is considering releasing part of the tapes or publishing a new, updated version of her autobiography.

“These are very personal tapes,” said Tamsen Harward, a manager at Chorion, the company that controls Christie's literary properties. “There are bits and pieces of the autobiography that could be reviewed, in light of listening to the tapes.”

And in a mystery that might have piqued the interest of one of Christie's fictional sleuths, only the final third of her life story can be heard on the recordings.

“We believe that, being a frugal woman, she reused the tapes,” Ms. Harward said, adding that Christie “clearly” did not feel the recordings had any historical value.

Her modern-day admirers may disagree. The tapes were dictated on a reel-to-reel recorder that was abandoned in the same box with the 27 reels of tape. With an occasional crackle in the background Christie can be heard talking about writing, about her characters and how she conceived them, with her tone varying from casual and meandering to crisp and professional.

“They're extraordinary,” said Laura Thompson, Christie's biographer. “Nobody sounds like that anymore. She's old England. She sounds like an Edwardian, like a gentlewoman. It's as though she's suspended in an early-20th-century world where the social order is intact, and murder is only conducted in a socially acceptable arena - arsenic in the crumpets, or something.”

Agatha Mary Clarissa Miller was born on September 15, 1890, to a wealthy American father and British mother. She married twice and kept a low profile, sometimes refusing to allow publishers to put an author photo on her books.

She wrote 66 detective novels, 163 short stories, 19 plays, 4 nonfiction works (including her self-titled autobiography) and 6 romantic novels under the pseudonym Mary Westmacott.