Philip Yancey writes about this many years of experience in publishing.

I had an enlightening experience with e-books in 2013. In April I finished the book The Question That Never Goes Away, based on my visits to three places of great tragedy. My traditional publisher wanted at least nine months lead time to publish it, the typical schedule for a new book, yet new tragedies such as the Boston Marathon bombings, tornadoes, and school shootings were occurring almost weekly, the very situations my book addressed. So I signed on for an Amazon-exclusive program to publish an e-book for 90 days before the hard copy book came out. Leaning on my friends for email lists, I managed to sell about 3,000 copies. On September 11 and Thanksgiving weekend I offered free downloads and 40,000 people downloaded the book! The moral of the story, as many have learned: things can quickly go viral on the Internet but it’s a tough place to generate income.