When Michael Rosen met Roald Dahl in a TV studio green room in 1980, both were successful children's authors - though Dahl was well on his way to becoming legendary. He had already written James and the Giant Peach, Charlie and the Chocolate Factory and Fantastic Mr Fox.
In the introduction to this biography, written for children with the aim of inspiring them to explore their imaginations and write their own stories, Rosen confesses he was excited at the notion of meeting someone whose books "millions of children loved".
Someone else was present at the meeting, however, who was even more thrilled: Rosen's son Joe, who was about five years old. Rosen says Dahl didn't pay too much attention to him but caught Joe's eye, summoned him to stand before him and demanded: "What's that growing on your father's face?"
When Joe replied "a beard", Dahl retorted: "It's disgusting. It's probably got this morning's breakfast in it. And last night's dinner. And old bits of rubbish, any old stuff that he's come across. You might even find a bicycle wheel in it."
Rosen's confused son was inclined to believe him. And that, Rosen points out, was the brilliance of Dahl. "When he spoke he did sound very, very certain - even if what he was saying was extraordinary, amazing, weird, fantastical or downright crazy."
The book itself is something of a fantastical contrivance, beautifully crafted to tell young people in simple but unpatronising terms about Dahl and how he became a writer. It details his early and sometimes unhappy years at school, complete with excerpts from letters to his mother; reveals his various pranks and stunts (the dead mouse in the sweet jar is still certain to make children laugh); his career in the air force and the plane crash that could have killed him; and later, his lavish lifestyle as a British spy in America. Dahl lived like James Bond. And almost everything he experienced would one day find its way into a story.
Rosen goes on to describe Dahl's own family, his writing hut in the back garden and his never-diminishing passion for food (his favourites were Norwegian prawns, lobster, caviar and "scrumptious roast beef").
But this is more than a book children will simply enjoy and treasure. Beautifully illustrated with sketches by Quentin Blake, it's designed to excite, inspire and prod young people to be curious about others. And above all, to prompt them to write their own stories. "What do you think?" Rosen asks his young readers, repeatedly.
He does add a caution about writers at the end of his introduction. "Now, a warning," he says. "It's impossible to write the whole, true story of anything. We always leave things out. We quite often put things in. Sometimes, no matter how hard we try not to, we change things. We tell the story in our own way."
This wonderful little biography is as delicious an introduction to what makes a writer as anything Dahl might have written.
Fantastic Mr Dahl is published by Puffin. Talk to author Michael Rosen online on 13 September at www.tes.co.ukwebchat.