How he walked the line. Johnny Cash’s darkest days are detailed in the recently published biography, The Man Who Carried Cash, written by Julie Chadwick.

The author spoke exclusively with Closer Weekly about her book — based off a trove of letters and documents found after Johnny’s longtime manager, Saul Holiff, committed suicide in 2005 — and revealed what she learned about The Man in Black.

MORE: EXCLUSIVE: Liza Minnelli Gushes Life With Mom Judy Garland Was Like Living in a "Land of Dreams"

In The Man Who Carried Cash, Julie writes that Johnny — who battled addiction years before his death at age 71 in 2003 —  hit a particularly low point in the 60s when he was downing 100 pills and guzzling a case of beer every day.

johnny cash - getty images

“The sheer amount of chaos was shocking,” says Julie. “I thought if I wrote it all down, people would think I was exaggerating.”

In her tome, Julie also covers Johnny’s extramarital affair with singer June Carter — and clears up some misconceptions about their relationship. As fans may recall, Johnny once crashed June’s Cadillac into a telephone pole while he was still married to his first wife, Vivian Liberto Distin. According to Julie, Saul did his best to cover up the accident in order to avoid scandal.

MORE: EXCLUSIVE: Loretta Lynn's Brother Says She Wants to Sing Again After Stroke — "That's All She Lives For"

“[In 2005’s biopic Walk the Line], they made it seem like the affair was much closer to the end of Johnny and Vivian’s marriage,” Julie tells Closer. “It seemed to start a lot earlier and carry on a lot longer than I thought.”

johnny cash june carter - getty images

Johnny eventually married June in 1968, two years after divorcing Vivian. Years later, he made a concerted effort to get clean and sober. In Julie’s book, she reveals that the musician wrote a 10-page letter to his manager about his problem with substance abuse. Says the writer, “That was part of his hitting rock bottom."

For more on Johnny Cash, pick up the latest issue of Closer Weekly, on newsstands now.

For more exclusive news from Closer Weekly, be sure to sign up for our newsletter!