The 76-year-old singer also claimed that the Thriller singer “couldn't cope with adult company at all” in his later years.
In an eye-opening excerpt from the book, Elton recalled how Michael had been “the most adorable kid you could imagine” but that he later began hiding from reality as he mental health issues worsened.
“God knows what was going on in his head, and God knows what prescription drugs he was being pumped full of,” he wrote.
“But every time I saw him in his later years I came away thinking the poor guy had totally lost his marbles.
The Rocketman singer continued: “I don’t mean that in the light-hearted way.
“He was genuinely mentally ill, a disturbing person to be around.
“It was incredibly sad, but he was someone you couldn’t help: he was just gone, off in a world of his own, surrounded by people who only told him what he wanted to hear.”
Elton also shared an uncomfortable story about Michael from the star’s later years.
He remembered how the Billie Jean singer had once withdrew himself from a party he hosted with his future husband David Furnish without eating any food.
Some time later, the pop icon was found in a cottage on the grounds where Elton’s housekeeper lived, playing video games with the housekeeper’s 11-year-old son.
“For whatever reason, he couldn’t seem to cope with adult company at all,” Elton added.
Michael died at the age of 50 on June 25, 2009, after suffering a fatal overdose of the anaesthetic propofol.
While the star still has millions of fans all around the world, his legacy has suffered following allegations that he molested children in a recent documentary entitled Leaving Neverland.