Harmon left NCIS on a strong note. The veteran actor had been playing Leroy Jethro Gibbs ever since its 2003 inception. After so many years starring in the show, Harmon rode off into the sunset seemingly because it was time to explore new opportunities. Fortunately, he was very satisfied with the open-ended conclusion of Gibbs' story.

"What has always drawn me here is the character I play and to keep it fresh and to keep it challenging," Harmon once told Entertainment Tonight. "Plot-wise, this character has taken the path that it did. I thought it was honest and OK with."

Once he finished the show, mainstream book publisher Harper Lee approached Harmon to write a book. But they wanted Harmon to write a book specifically about NCIS.

"Initially when [Harper Select] came to me and said, 'Would you write a book about the show?', I said, 'No,'" Harmon said to Parade.

Instead, Harmon saw potential in writing a book based on the actual NCIS organization, which was a callback to the show's original premise.

"I said I would be interested in possibly doing what I was told this was originally, which was every show was going to be based on a real case and part of that was my introduction to coming on board in the first place," Harmon added.

Harmon would eventually end up writing the book Ghosts of Honolulu with Leon Carroll Jr., a former real-life NCIS agent. Carroll also served as a technical adviser on the show, who Harmon discovered after doing research for the series.

"When I first googled NCIS, nothing came up," Harmon said in an interview with AARP. "These people did their work quietly, and nobody knew who they were. When you play a cop, you ride with a cop, to make it believable. Leon was just recently retired as the number one interrogator for NCIS, and before that he was in the Marines, which double helped me, because I was playing a Marine. There was never an interrogation I did on NCIS that I didn't talk to him about what was right, or what he would do. Leon and I had 20 years together to develop a friendship and a trust."

"He would pick my brain constantly," Carroll added about Harmon. "And of the 2,500 NCIS members, probably 1,000 visited NCIS over 20 years."

Unlike NCIS, which focused on an ensemble cast with Harmon as the lead, Honolulu told a story about one real life figure. Harmon and Carroll wrote about Douglas Wada, who was noted as the first Japanese American to work in Central Intelligence. During Wada's mission as a spy, Wada would also witness the attack on Pearl Harbor. Additionally, the book wouldn't be as focused on homicides as Harmon's NCIS, which was a common theme on the show.

"NCIS does more than investigate homicides," Carroll said. "Out of almost 460 episodes, I'd say 455 had a homicide. And David McCallum, God rest his soul, played a medical examiner, but NCIS doesn't have one. We rely almost 100 percent on county medical examiners and coroners."

"On the show, you had to have a dead body, so it became like a murder-a-week kind of thing, which is more a formula for a television series," Harmon added.

Despite Harmon's passion for Honolulu, however, he wasn't sure that meant he was going to make a career out of writing.

"I don't know whether this book will be successful," Harmon said. "We're hopefully coming out of the strike, so that's great for actors, and not just actors. You read [scripts], you say, 'I want to work with these people.' But I don't know what's next. Back when I started this career, I always thought it would be really nice to get to a point where you don't have to work if you don't want to."