(From Wikipedia):
The novel is set in the American Northwest. The main character is Mackenzie Philips, a father of five, called "Mack" by his family and friends.
Four years prior to the main events of the story, Mack takes his three children on a camping trip to Wallowa Lake near Joseph, Oregon stopping at Multnomah Falls on the way. Two of his children are playing in a canoe when it flips and almost drowns Mack's son. Mack is able to save his son by leaving his youngest daughter Missy alone at their campsite. After Mack returns, he sees that Missy is missing. The police are called, and the family discovers that Missy has been abducted and murdered by a serial killer known as the "Little Ladykiller." The police find an abandoned shack in the woods where Missy was taken, her bloodied clothing is found, but her body is never found. Mack's life sinks into what he calls "The Great Sadness."
At the beginning of the book, Mack receives a note in his mailbox from "Papa," saying that he would like to meet with Mack on that coming weekend at the shack. Mack is puzzled by the note - he has no relationship with his abusive father after Mack leaves home at age 13. He suspects that the note may be from God, whom his wife Nan refers to as "Papa."
I won't say more because you should read it. xD