If you were wondering just how close current Apple CEO Tim Cook felt to former Apple CEO Steve Jobs, well, here’s your answer.

The former attempted to give the latter part of his liver.

That’s the word from Becoming Steve Jobs: The Evolution of a Reckless Upstart into a Visionary Leader, a new Steve Jobs biography due out on March 24. Fast Company got its hands on a excerpt from the book, written by journalists Brent Schlender and Rick Tetzeli, and it explains how in 2009 Cook, distraught over Jobs’ worsening condition, took the steps to see if he could help.

One afternoon, Cook left the house feeling so upset that he had his own blood tested. He found out that he, like Steve, had a rare blood type, and guessed that it might be the same. He started doing research, and learned that it is possible to transfer a portion of a living person’s liver to someone in need of a transplant. About 6,000 living-donor transplants are performed every year in the United States, and the rate of success for both donor and recipient is high. The liver is a regenerative organ. The portion transplanted into the recipient will grow to a functional size, and the portion of the liver that the donor gives up will also grow back.

Apparently, Jobs shot Cook’s offer down immediately, saying, “I’ll never let you do that.”

More from the book:

“Somebody that’s selfish,” Cook continues, “doesn’t reply like that. I mean, here’s a guy, he’s dying, he’s very close to death because of his liver issue, and here’s someone healthy offering a way out. I said, ‘Steve, I’m perfectly healthy, I’ve been checked out. Here’s the medical report. I can do this and I’m not putting myself at risk, I’ll be fine.’ And he doesn’t think about it. It was not, ‘Are you sure you want to do this?’ It was not, ‘I’ll think about it.’ It was not, ‘Oh, the condition I’m in . . .’ It was, ‘No, I’m not doing that!’ He kind of popped up in bed and said that. And this was during a time when things were just terrible. Steve only yelled at me four or five times during the 13 years I knew him, and this was one of them.

So this is coming straight from Cook.

According to Cult of Mac, the new biography features interviews with Tim Cook, Jony Ive, Eddy Cue, Pixar’s John Lasseter, Disney CEO Bob Iger and Jobs’ widow, Laurene Powell Jobs. The book may be seen as a bit of a departure from the tone in Walter Isaacson’s best-selling 2011 bio Steve Jobs – which Cook himself maligns in the new book as doing a “tremendous disservice” to Jobs.