According to the publication, the "surprise success" of the book--more than 7 million copies sold since it was originally published by a start-up company launched by two friends in 2007--ranks it with other all-time self-publishing hits like 1931's The Joy of Cooking, 1970's What Color Is Your Parachute? and 1993's The One Minute Manager.

Forbes described The Shack--which has caused controversy for its depiction of the Trinity--as a "spiritual, often whimsical thriller." The magazine traced its rising sales, noting that they really took off after original publishers Windblown Media signed a printing and distribution deal with Hachette Book Group USA, "which last year had the highest titles-to-hits ratio in the business."

The "unexpected success" of the book, based on his own personal healing from past traumas, has transformed Young's life "to a point," the article said. Once working three jobs to make ends meet and having gone through bankruptcy, Young has seen "a reversal of fortune as sudden and dramatic as that of Job," earning $4 million.

"Self-publishing is a bit like playing the lottery," Forbes observed. "Most efforts remain vanity projects, but the lucky ones, often catching on by word of mouth, can hit the jackpot."