DES MOINES, Iowa—A year before it became a best seller, author Jeffrey Zaslow wondered if all the time he’d put into his book about the longtime friendship of 11 women would be wasted.

The women initially had been eager to talk about their enduring relationship, which began as teens in Iowa and continued decades later. But after reading a draft of what would become “The Girls From Ames,” some thought they may have been too open with Zaslow.

“After we got into it, they got more nervous and the book almost fell apart,” he said. “I read diaries and journals and learned things they didn’t want me to know. All gave me letters written to them by the others—there had to be a lot trust, which we lost a little and then gained some back.”

Zaslow, a Wall Street Journal columnist, kept the book on track by explaining the need to portray their lives and agreeing to edit out some personal details.

The result is a book chronicling the friendships of “girls” who endured life’s challenges long after their friendships first blossomed living in the small, college town among the cornfields of central Iowa.

The women, now in their 40s, are spread across the country and share tales of hardships and successes. Two have been diagnosed with cancer, one suffered the death of a child, two have been divorced and one mysteriously died at the age of 22.

Zaslow’s book, with 196,500 copies in print, remains at No. 24 on the New York Times Best-seller list after peaking at No. 3. It also rose as high as No. 4 on Publishers Weekly’s best seller list.

“It’s a really heartwarming book,” said Jan Weissmiller, owner of Prairie Lights Book Store in Iowa City, Iowa. “People, especially women, can really relate to the bonds they build with their friends.”

It all started with a column Zaslow wrote in 2003 about women’s friendships and how women are better at maintaining such relationships than men. After reading the column, more than 250 women contacted Zaslow to tell their stories about their friendships.

“I think every group of women has a story to tell and I was especially touched by a letter from Jenny from Ames,” said Zaslow. “I liked that they were in their 40s and from the Midwest.”

Zaslow called Jenny and quickly found that the women had stories that were universal but also many that were one-of-kind, exhilarating and haunting.

“I found myself spellbound as they talked to me,” Zaslow wrote.

Those talks led to “The Girls From Ames,” which chronicles the lives and friendships of Marilyn, Karla, Sheila, Kelly, Jane, Diana, Cathy, Sally, Karen, Jenny and Angela. After all these years, the group, down to 10 since Sheila died, still gather yearly for a reunion.

Zaslow, a Michigan resident who will be the grand marshal in the Ames Fourth of July parade, said the book required a lot of give-and-take and trust building.

The process started with phone calls between Zaslow and the women. He then flew around the country to meet with them and interviewed family members, old friends and ex-boyfriends.

It was Zaslow’s exhaustive research that left some of the women leery.

Kelly Zwagerman, a high school journalism teacher in Faribault, Minn., said she and her friends were eager to share their stories but became nervous when Zaslow sought more details.

“Jeff saw that some of those details were important and we didn’t see that at first. Why would people want to know that about us?” said Zwagerman, 46, described as the free-spirit of the group.