At her house on England's south coast, Mantel has the bones of a plan -- notes and sketches that she will stitch together to describe the downfall of one of English history's most equivocal and reviled characters: the blacksmith's son who carved his way to the top of Henry VIII's court and helped the king divorce his first wife, Catherine of Aragon, and execute his second, Anne Boleyn.

It's a challenging prospect for Mantel, whose first two Cromwell books -- the Man Booker Prize winner "Wolf Hall" (2009) and "Bring up the Bodies", which arrived in U.S. bookstores last week -- have won over critics and readers for their rich retelling, through Cromwell's eyes, of this well-trodden period of history.

"Wolf Hall" is being developed for a television miniseries written by Peter Straughan, who penned the screenplay for the 2011 movie "Tinker, Tailor, Soldier, Spy". A play is also in the works in Britain.

"If I get the third book right then in a sense my whole life will have come right," said Mantel, whose previous work has received critical acclaim but not the widespread popularity of the Cromwell books. "But if I don't, then I am going to see it as a failure. In my mind it is all one long project."

"The Mirror and The Light", expected in 2015, will mark the end of 10 years' research and the completion of an idea sowed nearly 40 years ago.

"I began writing in my early twenties and I thought, one day I will write a novel about Thomas Cromwell," Mantel said in a telephone interview with Reuters.

The final part of the trilogy will begin where "Bring up the Bodies" left off, with Anne Boleyn at the scaffold in 1536, and end with Cromwell's own execution four years later.

"His end is a mirror of his beginning. When we find him at the beginning of 'Wolf Hall' he is bleeding on the ground and he thinks he is going to die. When I wrote that I knew that I had the beginning but I also had an ending," Mantel said from her home in Devon.

INSPIRED BY HISTORY

Born in 1952, Mantel has written more than 10 works of fiction, including "A Place of Safety", set during the French Revolution, which, like her latest trilogy, concerns real historical characters.

But it wasn't until "Wolf Hall" that Mantel received popular attention at home and abroad. She humbly attributes it to the world's perennial fascination with the reign of Henry VIII that has spawned dozens of television shows, movies and plays. But critics also put it down to her fresh depiction of Cromwell, a man of the shadows brought into the light.

In plays such as "A Man For All Seasons", Cromwell is the calculating villain, responsible for the death of a saintly Thomas More, who stands up against Henry when the king divorces Catherine of Aragon.

But Mantel paints a more complex picture, a man ruthless in his politics but tender to his son; clever, thoughtful, sometimes humorous.

"I was surprised to find how much I admired him. My object wasn't to judge him, it was to get behind his eyes, challenge the reader to get in this man's shoes," said Mantel, who in her research studied original sources, including Cromwell's letters. "You don't spend 10 years of your life with someone you can't stand."

Work on "The Mirror and the Light" will begin next year, once the book tours, the readings, the talks -- all the fuss that success brings -- die down.

(Reporting by Edward McAllister in New York; Editing by Jill Serjeant and Dale Hudson)

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