Since its release on April 18, “Killers of the Flower Moon: The Osage Murders and the Birth of the FBI,” author David Grann’s recently-completed book about the Osage Reign of Terror murders, has been working its way up the U.S. best-seller lists.

Grann — who held local book signings Saturday, April 29, at The Water Bird Gallery and on Sunday, April 30, at the Osage County Historical Society Museum — calls the killing of Osage headright owners “one of the most sinister crimes in American history.”

“This is the rare case where there really is a conspiracy,” the New York writer said.

The 338-page hardcover hit bookstore shelves April 18 and is No. 15 on the Amazon best-sellers list for nonfiction books. Publishers Weekly puts “Killers of the Flower Moon” at No. 10 on its survey of top-selling nonfiction.

The New York Times survey of hardcover nonfiction best sellers awarded Grann’s book the No. 6 spot for the week ending May 14 — which marked the Osage murders’ book’s second straight week to be listed as a Times’ Best Seller.

Appearing for the 22nd week as the NY Times’ Best Seller list is Grann’s previous release, “The Lost City of Z: A Tale of Deadly Obsession in the Amazon,” a 2009 book detailing British Colonel Percy Fawcett’s Victorian-era quest to find an indigenous civilization in the remote jungles along the Amazon River. It is currently No. 4 among Times’ non-fiction paperbacks.

A movie version of The Lost City of Z came out in April 14. Through Sunday, May 7, the Amazon adventure film had achieved domestic box office totals of more than $6.6 million. The film has made another $2 million worldwide. It’s scheduled for a July 11 release on DVD.

‎Initially, Brad Pitt had been cast to portray Fawcett. Later, the part was assigned to Benedict Cumberbatch, one of the stars of 2013’s locally-filmed “August: Osage County.” When scheduling conflicts eliminated both those actors, the role went to Charlie Hunnam. Pitt’s Plan B Entertainment handled the production duties.

In May 2016, movie rights for Grann’s book on the Osage murders sold at auction for $5 million — which was reported to be the record high for a not-yet-unpublished work.