Jonathan Eig’s new biography of Martin Luther King Jr was only published last week, but it has already been hailed by the Washington Post as “the most compelling account of King’s life in a generation”. The documentarian Ken Burns described it as “kind of a miracle” and the New York Times declared it “supplants David J Garrow’s [Pulitzer-winning] 1986 biography, Bearing the Cross, as the definitive life of King”.

In a remarkable act of generosity, Garrow opened his files to Eig and acted as his consultant. Garrow now agrees with other critics, calling Eig’s book “a great leap forward in our biographical understanding” and “the most comprehensive and original King biography to appear in over 35 years”.

Eig is a former Wall Street Journal reporter who has written five other highly regarded books, including bestselling biographies of Lou Gehrig and Muhammad Ali. This week, Eig chatted about how his book on King came about and what he hopes readers will take from it.

The Guardian: I read somewhere that the new book came out of your work on Ali.

Eig: Yeah, it was completely organic. I was interviewing people who knew both of them and every time they would start talking about King, I would just get more curious. So I felt like I already had their phone numbers. I could call them back and get another meeting and this time talk about King. And I could do that before they got any older.

The Guardian: When I wrote The Gay Metropolis I started with the oldest people I could find. Did you do that?

Eig: 100%. It was like actuarial tables: factor for age and health and go after those who are the most frail. I hate to be crude about it, but that’s exactly what I did. Basically I was calling everybody all at once.

The Guardian: How long did this one take?

Eig: This one was six years. That’s full-time work, like 60 hours a week for six years.

The Guardian: You had access to thousands of FBI files that weren’t available to previous biographers. How did that come about?

Eig: I got somewhere between 5,000 and 10,000 new documents. Donald Trump signed an order to release documents that were gathered during congressional hearings on JFK’s assassination. And I think accidentally that also led to the release of all the MLK FBI stuff, because the Church committee [a 1975-76 Senate panel on government intelligence activities] investigated them both.

I really think Dave Garrow was the only one who went through every file. I went through a lot of them and Garrow was kind of like the first reader and he would tell me what was important and I, of course, looked through a lot on my own. But I don’t really know that too many other people were out there looking at this stuff.

The Guardian: You did more than 200 interviews. Why were there so many people who knew King who were much more forthcoming than they had been before?

Eig: Because they were older and because Coretta [Scott King, King’s wife] was gone. They were more comfortable saying things that they wouldn’t have said before. Certainly when it came to talking about Dorothy Cotton [one of King’s mistresses], people were really reluctant to say anything while Coretta was alive.

The Guardian: I always tell my young friends writing a great book is all about what you leave out. Do you agree?

Eig: (chuckling) Yeah. Even at 600-something pages! I left out a lot. At one point – I’ll be honest – I asked Colin Dickerman [his original editor] if I could do a three-volume work. I wanted to do one from childhood to Montgomery and then from Montgomery to maybe Selma and then Selma to death. Wisely, Colin disabused me of that idea. I’m trying to give the reader not just a good book but a readable book. I told my wife, I want people to cry at the end of this book – and they’re not gonna cry if I’ve put them to sleep!

The Guardian: What do you know now that you didn’t know when you wrote your first book, about Lou Gehrig?

Eig: It took me a couple of books to figure out that journalists’ archives are really valuable … When you find a good interview a journalist did with one of your subjects, go to his archives and see if the notes are there, see if the tapes are there.

I got David Halberstam’s notes from his interview with King and he describes King taking his kids to the swimming pool and his daughter falls and scrapes her knee. And King grabs a piece of fried chicken and rubs it on her knee and says, “You know, chicken is the best thing for a cut.” It’s just a sweet little moment that didn’t make Halberstam’s story. But it was in his notebook.

The Guardian: You describe King as one of America’s founding fathers. I’d never seen that before.

Eig: Yeah. It was my idea. It was inspired somewhat by reading some of the 1619 Project. They talk about the idea that Black activists were seeking to force the country to live up to the words of the founding fathers. And that’s what kind of triggered it for me. I think you can make an argument that King more than anyone else is a founding father. He’s trying to create the nation as it was meant to be.

The Guardian: The great Texas journalist Molly Ivins said something similar: “There’s not a thing wrong with the ideals and mechanisms outlined and the liberties set forth in the constitution of the US. The only problem is the founders left a lot of people out of the constitution. They left out poor people and Black people and female people. It is possible to read the history of this country as one long struggle to extend the liberties established in our constitution to everyone in America.”

Eig: Yeah, I, I like that.

The Guardian: What would you most like people to feel from reading your book?

Eig: I hope people see King as a human being and not this two-dimensional character we’ve made him into since he became a national holiday and monument. [They should know] he had feelings and suffered and struggled and had doubts, because I think that makes his heroism even greater.

I certainly want people to appreciate just how radical he was. A lot of people reduce him to this very safe figure who was all about peace, love and harmony. But he was challenging us in ways that made a lot of people uncomfortable, which is partly why the FBI came down on him the way they did.

The Guardian: The thing that I think is probably most forgotten about him is that he was as anti-materialism as he was anti-militarism. Would you agree?

Eig: That’s right. And it drove Coretta crazy because he would never even buy nice stuff for the house. And of course he left no money behind when he died. So he took it really seriously.

• King is published in the US by Farrar, Straus and Giroux