For help with his new White House thriller, "The Fifth Assassin," author Brad Meltzer won critical advice and tips from two sources that provided shocking details about the nation's four presidential assassinations and how modern presidents live knowing a bullet could come any day.

Meltzer tells Secrets that for his new book, officially out Wednesday, former President George H.W. Bush, who just left a Houston hospital, provided hints of what life is like inside the White House security bubble.

But even more revealing was an offer to visit a little-known Army museum that houses pieces of Abraham Lincoln's skull, the bullet that killed him and the bones of assassin John Wilkes Booth.

"The Fifth Assassin" follows a killer who picks traits of the four previous presidential killers for his mission, finding surprisingly similarities among them. And that was Meltzer's experience researching the book.

"Former President Bush was amazingly helpful with the research, even putting up with my questions about whether presidents are afraid when they're in the White House," said Meltzer, a New York Times best-seller.

"But the best news comes from the research on the assassins themselves. While doing the research, I came across a long lost tattoo -- on an actual piece of skin -- that might've belonged to one of the assassins. It was in an old box in an Army museum that holds the bones of John Wilkes Booth, the actual trigger finger of one of the other assassins, and even pieces of the skull of Abraham Lincoln."

He even had a source reveal details about the long-suspected secret tunnels under Camp David. "Just don't tell anyone where the tunnels come out," he urged Secrets. "That'll ruin chapter 97 of the book."

His theory that assassins and potential political killers have similarities and even copy each other may seem fanciful, but past readers know that Meltzer's research is air tight. He provided us with one example of what his research uncovered:

"Back in 1994, a man named Francisco Martin Duran tried to kill President Bill Clinton by firing 29 shots at the White House. But on his drive from Colorado to Washington, did you know he stopped in Dallas, Texas, passing the Book Depository (from where Lee Harvey Oswald shot former President John F. Kennedy] and than when he got to D.C., he even stayed at the Hilton Hotel where John Hinckley shot [former President] Reagan?" he said.