The book, which debuted earlier this month, is now number one on the New York Times best sellers list.

It introduces former Army Ranger John Puller, now the “best military investigator in the U.S. Army’s Criminal Investigative Division,” who is assigned to solve the particularly brutal murder of Army Colonel Matthew Reynolds and his family in a small West Virginia mining town.

Baldacci said he based his hero on CID agents he’s known – agents who are called in when a crime involving the military happens in a civilian environment.

“These guys tend to be very tenacious,” the author said. “I became interested in how the military would work in the civilian justice sector. The American people see a lot about the military soldiers and the Pentagon and the war. I wanted to take a soldier who had seen all that war and bring him back to work on domestic ground.”

Puller is a haunted man. A career Army veteran, he’s lost men in Afghanisitan, driving him close to the edge.

“As horrific as the murders of the Army officer and his family were, they couldn’t come close to what Puller’s seen in battle,” Baldacci explained. “He’s got a laser beam focus. When he misses something, people die. He can’t accept that. If he becomes unfocused, the edges would start to swallow him. He can’t allow that to happen. He’s like a machine in some ways, but right now, that’s the only way he can survive.”

Creating a new character has its own challenges.

“I really tried to focus on making John Puller the kind of guy who’s going to lead you through a story,” he said. “He doesn’t care about upsetting superiors or getting local cops mad at him. He needs to find out who killed these people.”

Puller’s father is a military hero. His brother, also an officer, is locked up for treason.

“For a military person, that’s an extraordinary amount of baggage – a legend on one side, a traitor on the other,” Baldacci said. “Having a father who was a major general is not always a good thing.”

Puller, however, decided to build a career in the Army. He didn’t attend West Point like his father and brother. He’s made his own way, on his own terms.

There are more John Puller books planned.

“This was the first book I’ve written where I sat down and knew this was going to be a series,” said Baldacci, whose book sales have reached over 110 million. “We’re sending it [‘Zero Day’] out to Hollywood this week. We’ll see what happens.”