The classic children's tale "Watership Down" was born on a summer day in the mid-1960s. Richard Adams, a civil servant with an Oxford education, took his two young daughters for a walk in the British countryside. They wanted to be entertained, and he obliged.

"He began the story with the words, 'Once upon a time, there were two rabbits named Hazel and Fiver,'" recalls his younger daughter, Rosamond Mahony, who was around 6 years old at the time.

The story revolves around a group of rabbits who leave their warren and embark on a perilous journey in search of safety, drawing upon Adams's knowledge of Greek tragedy and the English countryside where he grew up.

"He was making it up as he went along," Mahony explains. "But when we got home, the story was so long that he hadn't even finished it. So, it had to be concluded on the way to school the next morning."

Since then, "Watership Down" has teetered between children's fantasy and adult literature. The release of a graphic novel this month—a genre enjoyed by both young and old—further deepens the discussion of who this story is for and whether children's books were ever exclusively intended for children.

"He just said that this book was written for anyone who wanted to read it, and that could be anyone from someone so small they could hardly hold the book to someone so old they could barely see the print," says Mahony.

The story of the rabbits turns out to be multi-layered. Adams treated his children not as youngsters but rather as peers. He took them to the opera and read them Shakespeare. In fact, the summer trip was meant to allow him to watch a play by the Royal Shakespeare Company in Stratford-upon-Avon.

He also used his vast imagination in their interests, which, according to Mahony, were not fully explored as a member of the country's vast bureaucracy.

Her older sister, Juliet Johnson, sometimes suggests, however, that their father could be exhausting.

"There was always noise and shouting. And, you know, he could talk about various things until the cows came home," she says.

After their father finally finished telling this story, the girls advised him to write it down. Eventually, he did, refining it along the way and regularly consulting with his daughters. By that time, Johnson was away at boarding school.

"Somewhere around here, in a tin, I have a box of letters where he would get stuck on things," Johnson recalls. "He'd say something like, 'I haven't decided yet whether to let Bigwig die in the snare or not.' Or, you know, 'I need to come up with a few more jokes for Bluebell.'"