Thousands of people in the Denver metro recently found the same book in their mailboxes.
It's titled "The Great Controversy", and dozens of people turned to 9NEWS with the same questions: who wrote it and who sent it?
Inside, there's a card that says the book is a gift. That card led us to Dwight Hall, CEO and founder of Remnant Publications, a Christian publication group based in Michigan.
Hall says his publication group is doing the bidding of people who've read the book and want to share it with others. He says the book was written by a founder of the Seventh-Day Adventist Church.
"It's not to try and force any religion down anybody's throat," Hall said.
In 363 pages, the book walks readers through persecution of Christians. By the end, "it talks about prophecy and how things end according to the scriptures," Hall said.
Hall said "Controversy" has been distributed for decades. Using their network of fans of the book from around the country, Remnant Publications raised money to send it to thousands of homes for free.
Remnant Publications used zip code mailing, which the U.S. Postal Service says is legal.
"The First Amendment allows freedom of speech," David Rupert said, the postal spokesman for Colorado.
Hall said using the guidance of the book's fans, certain zip codes were picked and Remnant Publications sent enough books to be delivered to those homes on that mailing route.
"Restaurants will do that. Dry cleaners will do it. Political campaigns will do that," Rupert said.
According to Hall, around 10 million copies have been delivered across the country over the last 3 1/2 years. Hall said it's up to people to decide what they want to do with it.
"Give it to a friend, I guess throw it away; it's yours!" Hall said.
Hall says his group sent the book to nearly half a million homes in the Denver area last week -- a mass mailing that cost around $750,000 to print and deliver.
