The late great Johnny Cash is lucky he found the Lord and true love — because he ­surely would have self-destructed otherwise. According to a cache of ­secret correspondence — hidden for decades by his former manager, Saul Holiff — The Man in Black was deep into drugs, booze, unrestrained lust and criminal behavior  far beyond anything previously reported!

Holiff, who committed suicide at age 80 in 2005, had tucked away a trove of personal letters, audiotapes and diary entries in a storage locker. Opened after Holiff's death and now revealed in a shocking new book, "The Man Who Carried Cash," by Julie Chadwick, the evidence proves the country legend was a truly tortured soul.

"Johnny's drug problem was far more intense than as portrayed in films like 'Walk the Line,'" says Chadwick.

Holiff became Cash's manager in 1960 and steered his client toward superstardom. Along the way, however, he bore witness to Johnny's dark side and the downward spiral that threatened to end his career — and his life!

Once Johnny even smashed his then-mistress June Carter's fancy Cadillac into a phone pole, knocked out his teeth and was refused treatment from doctors who believed the injured star was "bad news."

Another time he was found overdosed and "virtually dead" on the floor of his motor home after a show in Toronto in 1966.

Fans of the "Folsom Prison Blues" singer know he was no angel in his youth, but the extent of his wild ways haven't come to light before. Johnny may not have shot a man in Reno just to watch him die, but he did shoot up a hotel in Canada just for fun! When capers like flushing a cherry bomb down a toilet and trashing rooms grew boring, Johnny and two men in his entourage ran into the hallway, firing Colt .45 pistols!

Perhaps his most damaging act occurred in June 1965, when a drugged-up Johnny accidentally started a fire in California's Los Padres National Forest that destroyed more than 500 acres, displaced 49 endangered condors and almost got him killed. Though Johnny blamed a faulty exhaust system on his truck for the blaze, his nephew, Damon Fielder, who was present, believed the star started a fire for warmth and was too stoned to notice it had gotten out of control.