The much-maligned first wife who inspired Cash's 'Walk the Line'

The letters, written while Cash was serving overseas in the US Air Force, form the basis for a new book that could go some way to correcting the image of Liberto, his wife for 13 years, who family members believe was poorly treated in the recent Oscar-winning film of the singer's life.

The title of the book itself is a poignant reminder to fans that Cash's 1956 hit "I Walk the Line" was written as a fidelity pledge to Liberto and not his second wife, June Carter Cash, played by Reese Witherspoon in the film of the same name. Family members, including the singer's daughter Kathy Cash, have complained that the biopic misrepresented her mother who gets precious little screen time in comparison to Witherspoon's more glamorous character.

I Walked the Line, co-written by Liberto Distin before her death last year, will hit bookshelves early next year and is said to show Cash during a more clear-headed period of his life before being beset by drug problems.

Ann Sharpsteen, the other co-writer, said: "The letters really reveal the real man, unclouded by drugs. Letters were his dreams, fears, a variety of subjects, fidelity, alcohol, faith. It's like reading someone's diary."

The biopic, which starred Joaquin Phoenix as Cash, showed a musical genius beset by doubts and crippled by drug dependency while it offered little more than a glimpse of the singer's first marriage. Cash and Liberto were married in 1954 and had four daughters. Liberto reportedly approached Cash shortly before he died in 2003 to ask permission to incorporate their letters in a book. Kathy Cash has said that her father was immediately enthusiastic. "He said, 'Vivian, if anyone on this whole earth should write a book it should be you,'" she said.

When Liberto died last year the book was already in gestation with co-writer. I Walked the Line, to be published on Valentine's Day, draws on the letters sent daily to Liberto by Cash over four years after he was stationed in Landsberg, Germany, as a radio intercept operator in 1950.

Sharpsteen said the book helped close wounds for Liberto. "She had never gotten over Johnny, so it was a journey of healing."

The book may also ease any hurt felt by the four daughters in the wake of the recent film, which earned Witherspoon an Academy Award. Kathy Cash publicly voiced her dismay last November. "My mom was basically a nonentity in the entire film except for the mad little psycho who hated his career," she said. "She loved his career and was proud of him until he started taking drugs and stopped coming home."

James Mangold, the film's director, has denied that he ignored Cash's earlier years or his first family. "We interviewed Kathy as well as her mother, Vivian, in the making of the movie," he said recently, "but it's very hard for kids growing up in a family seeing stuff like this dramatised in front of them."

Cash began to drift from his first wife after his career started to take off and increasingly committed him touring the country. It was during the tours that he began drinking and taking drugs. And while on the road he also first met June Carter, who was herself divorced.

The book will be published in the US by Scribner, a unit of Simon & Schuster. In the meantime, popular fascination with Cash - sometimes called the "Man in Black" because of the monochrome uniform he always wore - seems only to deepen. Earlier last week, a new Cash-inspired musical, Ring of Fire, opened on Broadway in New York. Built around two dozen of Cash's songs, it makes no attempt to chronicle his life but is billed only as a celebration of his genius. Prospects for it may have been dented by less than complimentary reviews - The New York Times panned it as "an artificially sweetened candy necklace".