But he believes there has never been a definitive biography written about Ali. And he's out to correct the oversight in "Ali: A Life," a tome that will span more than 600 pages when it is officially released Oct. 3.
"I felt like this was probably the biggest and best subject a biographer could ever ask for," said Eig, who has also authored books on Lou Gehrig, Al Capone and Jackie Robinson. "I wanted to write the greatest biography of all time because to me it's the greatest subject for a biography you could ever have."
It's hard to argue that, since Ali himself was The Greatest. The way Ali's life played out both inside and outside the ring has provided prime fodder for writers from the time Ali won gold in the 1960 Olympics in Rome as Cassius Clay.
Indeed, just in time for the anniversary of Ali's death, there's another new Ali book that deserves attention. Former Sports Illustrated writer Leigh Montville wrote "Sting Like A Bee," a fascinating account of Ali's fight against the U.S. government from 1966 to 1971.
Montville sets the tone for his book in the introduction, with a scene from Ali's funeral procession in Louisville. Ali's casket was in the back of a Cadillac hearse, fitting because Ali loved to buy flashy new Cadillacs and cruise about town.
The one carrying him to his final resting place had to stop several times along the way to clear flowers tossed onto the windshield. Ali was celebrated as larger than life, but Montville writes that he was very much a human being.
"This was the guy who drove the Cadillac," he wrote. "Not the saint who rode in the back."
Eig says he was also careful to balance the legend that is Muhammad Ali with the person who was Muhammad Ali. Every man has his flaws, and Ali had his own, including things like his call for separation of the races in the 1960s and the cavalier way he treated the women in his life.
"You want everything to be neat and perfect, but that's not real life," Eig said. "People are complicated, and Ali was very complicated."
Eig interviewed more than 500 people for his book, and read almost every book written about Ali. That included the 1991 biography by Thomas Hauser that was done with Ali's cooperation and other reads like Norman Mailer's "The Fight," an account of the legendary 1974 "Rumble in the Jungle" with George Foreman.
Everybody he approached, he said, cooperated to some extent, though Ali's widow, Lonnie, declined a full interview. Some of his children did do interviews, as did Foreman, Larry Holmes, fight doctor Ferdie Pacheco and Ali business manager Gene Kilroy.
Some of the fighters Ali faced, Eig said, had the most interesting takes on their times with the champ.
"George Foreman still believes that the fight was fixed and he was drugged before the fight," Eig said. "Guys like Larry Holmes, guys who fought, never get over the fact that they're warriors. Larry Holmes started out saying all these nice things but once he got comfortable he said Ali was stupid for allowing himself to get hit. 'Rope a Dope,' he said, only Ali was the dope."
The number of times Ali did get hit will be documented, with Eig working with Compubox statisticians to watch tapes and record each punch. There will also be revelations through a study by speech scientists on when Ali's issues with Parkinson's began and how it affected him as an active fighter in the 1970s.
What there won't be, he said, is fawning praise for Ali on every page.
"What he said and did is important and changed the world," Eig said. "We don't need to romanticize and deify him."