I read it (because my mother-in-law gave it to me for Christmas, hoping to inspire me to go to church) and, yes, it is self-published. It's an amazing success story for a self-published book, but I think Christian fiction, in general, has more success with self-publishing than other genres. Christian writers have an established network for marketing their books through churches and other religious organizations.
I thought the first few chapters (before he arrived at the shack) were very well-written and evenly paced. I was intrigued even though I resisted the idea of "getting into" a religious novel (It's not that I have a problem with Christian fiction, but it appeals to me about as much as Romance. Other people love it, but it's just not what I read.). But I started to lose interest when the main character met with God. The theology was very interesting from that point on (I've never believed in hell and this "God" tries to explain to the main character that hell is a human concept--that a merciful God would not eternally punish his/her children.), but the main character literally stays at the shack from then on. For about three-quarters of the book, he simply talks to God, talks the Holy Spirit, chats with Jesus and then chats with God again. I needed a little action in there. And I really needed Jesus to quit grinning and chuckling so much.
I think it could easily have gotten published through more traditional means, but I wonder whether it would have been so successful. This author's story--the fact that his target audience was his grandchildren and that he wrote it simply as a therapy of sorts--is part of its appeal. I am not generally a fan of self-publishing but, in this author's case, he clearly had the right combination of factors working in his favor.
I think it will be a while before we see another self-published novel on the best seller list.