BOSTON (CBS) — Roseann Sdoia and three people who now make up her “inner circle” didn’t even know each other before the Boston Marathon bombing. But the bond they share now is as close as any they have ever known.

Sdoia was gravely injured in the second explosion. Shores Salter, a Northeastern University student, was running away from the blast when he heard her cries for help. He turned around and raced to her side. Salter, who had no medical training beyond what he needed to teach a tennis lesson in high school, held the tourniquet that kept Roseann from bleeding to death.

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Roseann Sdoia (WBZ-TV)

Shana Cottone, a Boston Police officer, got right in Sdoia’s face—asking questions and trying to keep her alert. She also commandeered the police van that rushed Sdoia to Mass General Hospital.

Firefighter Mike Materia also helped care for Sdoia at the scene and then rode with her to MGH, holding her hand the whole way.

That’s where Sdoia’s new book, Perfect Strangers begins. But it goes on to describe how the lives and history of these four friends now make them inseparable.

Each describes the critical role the friendship played in helping them recover from their emotional wounds in the aftermath of the attacks. Sdoia tells WBZ’s Lisa Hughes, “Just because your life changes… it can change in a good way, in a positive way. And for me, it’s been positive having these people in my life.”

Salter, for example, felt lost in the two weeks between the bombings and his reunion with Sdoia. In Perfect Strangers, Sdoia writes about the moment she and Salter finally saw each other at MGH.

Salter’s mother told Sdoia that the reunion saved him.

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Perfect Strangers by Roseann Sdoia (WBZ-TV)

“It definitely helped me at that time and continues to help me every day,” Salter says. “We were all there for each other and since then, we’ve all picked up the pieces and moved on … together.”

Cottone describes April 15, 2013 as “the worst day of my life.” She wondered if she could continue working as a police officer.

Watching Sdoia’s recovery helped her focus on something other than her own challenges.

“It was good. It was therapeutic,” she said of her friendship with Sdoia, Materia and Salter. “I don’t know if I would have gotten back up on my feet the way I have had I not had them.”

And Materia became Prince Charming. He was a regular visitor when Sdoia was in the hospital and by her side when she left. He’s still there now, as her fiancé. They have adopted two dogs, Sal and Lou, and are planning their wedding.

They will all be at this year’s Boston Marathon. Salter is running his first marathon for The Heather Abbott Foundation to buy prosthetics for amputees. He says it will help him reclaim Patriots Day which, he always believed was one of the best days of the year.

Sdoia says she will be waiting for him at the finish line and cannot wait to share the accomplishment.

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Perfect Strangers will hit bookstores on March 28, 2017.