Mark Harmon, known for his role as Leroy Jethro Gibbs on the CBS series NCIS, and Leon Carroll Jr., the show's technical advisor, have co-written a book that echoes the beloved police procedural in more ways than one. 

Ghosts of Honolulu: A Japanese Spy, A Japanese American Spy Hunter, and the Untold Story of Pearl Harbor, out this fall, is a historical account of the Japanese and American intelligence agents in World War II. The book sheds light on "the incredible high stakes game of naval intelligence," says its publisher in a statement."These are the real stories from those who were there," Harmon tells PEOPLE over email.

"It's real and it's our history."

Ghosts of Honolulu connects the lives of Douglas Wada, the first Japanese American Office of Naval Intelligence (ONI) agent, and Takeo Yoshikawa, a Japanese spy assigned to gather information on the United States. Both men had crucial roles in "the world-changing cat and mouse games" between Japanese and U.S. military agents in Hawaii before World War II, which climaxed during the 1941 Pearl Harbor attack. The book also details the moral conflict many Japanese American officers faced as they were torn between defending their country and their communities during the war.

Harmon and Carroll Jr. were not only inspired by the story's historical significance, but also ONI's impact on the real life Naval Criminal Investigative Service. Previously called the Naval Investigative Service, the NCIS was developed by ONI, and changed its name in 1992 to "emphasize the criminal mission of the agency," according to Harmon. For 18 years, he played Special Agent Gibbs on a fictional team within the organization, before leaving the role in 2021. 

"On the show we used to use the fact that no one knew who this agency was or what they did as a point of humor, but for those doing this difficult job for real, there was nothing funny about it," the Emmy-nominated actor says.

"To provide these men and women a chance to tell their story, their REAL story, has been both fun and educational for me."

Carroll Jr. says that working with Harmon on the book has been "seamless.""Through an 18 year relationship, Mark and I have developed a deep appreciation of what we can do together as a team," he says. "Considering different options and approaches to storytelling is similar, whether it is a book or a television show."Carroll Jr., a former NCIS special agent, also found similarities to his previous career throughout the writing process. He was particularly impacted by the story of Officer Wada, a dedicated operative throughout his time at ONI.

"He was using many of the same operational methods that I was trained in some 40 years later," Carroll Jr. says. "I could definitely relate to the challenges he faced maneuvering in an environment where even his co-workers were not sure about what he really did."

For Harmon, whose acting career ranges from TV classics like The Love Boat and Moonlighting to political and medical dramas The West Wing and St. Elsewhere, it feels "weird" to now add author to his long list of credits. (He was also named PEOPLE's 1986 "Sexiest Man Alive" and was a cover story in 2019.)

"Actors tell stories, so that part is comfortable," Harmon says. "[But] I have enjoyed great teachers and help, [and] the idea to grow and learn something new every day."