When she was 12 Brooke Hayward, the eldest child of actress Margaret Sullavan and agent-producer Leland Hayward, wrote a children's book titled The Riders of Red Devil. It was about twin boys who rode together on the same horse and won the Kentucky Derby. The book was accepted by Putnam's, but Brooke's mother, who wanted to shield her children from the demands of fame, would not allow it to be published. "If you ever write again—and you will," Sullavan told her daughter, "write about something you know."

She has done so—a portrait of her unusual family called Haywire. As a result, Brooke Hayward, a sometime actress (a 1961 movie called Mad Dog Coll) and model, a two-coast dilettante with a well-known name but no distinctive persona, has become Brooke Hayward, best-selling author.

"I wrote in a bedroom crowded with ghosts," Hayward says. "My mother would disapprove, and my father would be horrified. The moral of my book is that you pay for everything. They were rich, accomplished, famous and beautiful. We were drowned in privilege, yet it ended in all this hideous tragedy."

After a bitter divorce when Brooke was 10, the family was torn apart. The deaths of both her mother and sister were probably suicides, and her brother Bill has spent about three years in mental institutions. Her father—who represented such celebrities as Garbo, Hemingway, Garland and Astaire—died in 1971 after a series of strokes.

Writing the book was difficult, says Brooke. "I would weep. I had never confronted my parents with the true feelings I had for them, and I had certainly never expressed the depth of my feeling for my mother, being too selfish to try when I should have." The book became a compulsion. "My friendships were precarious. I ignored my child's education and let my life slide. I'd go to bed fully dressed, get up about 5, stagger to the typewriter and start right in again." Some days she produced as little as one sentence.

To help her memory, Brooke interviewed some 30 family friends including the three Fondas, Jimmy Stewart and Josh Logan. Most of them are quoted at length in the book. "My parents were very unusual people, but it was more valuable to have other people say that than me."

Brooke was born in Los Angeles 39 years ago. She, her younger sister, Bridget, and brother Bill lived in a separate house with their nanny on the Hayward property in Brentwood. Then Sullavan, who hated Hollywood, took the family east to Connecticut. Brooke met her first husband, Michael Thomas, a Yale art history major, while she was at Vassar. They were married in 1956 and divorced four years later. Son Jeffery, 20, is a junior in drama at the University of California at Santa Cruz, and William, 19, is a sophomore film major at Reed. Brooke feels that she and Michael, a stockbroker, avoided the mistakes her parents had made during and after their divorce. "We put up a united front. Our sons knew that they had been the product of a relationship between two people who cared for each other."

Actor Dennis Hopper was Brooke's second husband. She was on Broadway in Mandingo in 1961 when Hopper joined the cast. "He set out courting me immediately," she says. "Dennis is an extremely interesting character, for whom I have some regard—although he terrified me." Their marriage lasted until 1968, the year Dennis directed Easy Rider, with Peter Fonda and Bill Hayward among the producers. Hopper slipped underground after the movie's phenomenal success. He never sees Brooke and, she says, "is the worst father in the world" to their 15-year-old daughter, Marin, who goes to school in Arizona.

Hayward, who has "a small income" from a trust set up by her mother, says she is "getting older and more eccentric by the minute, and used to stumbling around by myself. I have a series of splintered relationships. Why should I get married again? It's a miserable compromise at best. But I believe in marriage and still have fantasies about it."

Although Hayward claims she "deliberately wrote Haywire so it couldn't be converted into a movie," she has been persuaded to approve a mini-series for TV. Her brother Bill will be producer. Bantam paid almost $350,000 for paperback rights, and Brooke will get even more than that from TV. "I wrote a book," she says, "and frankly that's what I'm interested in selling. The TV thing will be on the air within the year and will greatly affect the paperback sales."

As for Hayward, she has bought a new house in Beverly Hills and is winding up an exhausting promotion tour. After that, to make sure she doesn't go haywire herself, she'll head for Europe and a long vacation.