John Grisham decided to write this book after he read a rather long obituary in the New York Times. He realized there was a story to tell and decided to tell it. It was probably a bigger undertaking than he had realized, but he persisted. It seems like it took him 18 months, whereas he writes most of his yearly books in about 10 months.
Grisham discusses this book on The Innocence Blog 5/21/2008, including a video.
The book told the stories of Ron Williamson and Dennis Fritz, who were both convicted of murder in 1988, then exonerated and released in 1999. Many people were involved in having their convictions reversed, including The Innocence Project, a national litigation and public policy organization dedicated to exonerating wrongfully convicted people through DNA testing and reforming the criminal justice system to prevent future injustice. The Innocence Project gives outlines of all those who have been exonerated after their convictions and imprisonment. This list goes back to 1989. As of this date, 234 wrongfully convicted people have been exonerated.
One of the key characters in this true story was Pontotoc County District Attorney Bill Patterson. Even though DNA testing freed Williamson and Fritz and pointed to the real murderer, Patterson continued to believe Fritz and Williamson were involved in the crime even though the mother of the victim totally accepted the facts as they were revealed by the DNA.
I recommend Grisham's book, but be prepared that it may have an unexpected affect on you as you realize what you are reading is not fiction.