In I Will Always Write Back: How One Letter Changed Two Lives, the power of writing brings together two teen pen pals in an unlikely, enduring friendship. The nonfiction book, published by Little, Brown Books for Young Readers next month, begins with 12-year-old Caitlin, a typical American preteen from Hatfield, Pa., whose seventh-grade class is corresponding with kids across the world. Asked to choose a country, Caitlin picks Zimbabwe, which sounded the most far-flung and exotic. Her first letter reaches Martin, who lives with his family in a poor suburb of Mutare, where his father works at a local paper mill.

Through six years of correspondence, readers watch Caitlin and Martin’s friendship deepen; at the same time, the political and economic situation in Zimbabwe deteriorates. They continue to write, Martin sending Caitlin the only photo of himself that he has. As they open up to one another, Caitlin is distraught to learn that necessities she easily takes for granted, like clothes and school, are almost out of reach for her friend.

The letters gradually give way to care packages containing trinkets and, sometimes, Caitlin’s allowance. While a one-dollar bill or the odd T-shirt are of little significance to Caitlin, the money means, for Martin, more food for his family; the clothes give them bragging rights in their community. When Caitlin later sends $20, the relative fortune enables Martin to pay his school fees and continue his education. Later, Caitlin’s family helps Martin find college scholarship opportunities in the U.S., which brings him to Villanova University, and later to an MBA at Duke University. Eighteen years after the first letter, they are still the best of friends.

Writer Liz Welch came on board to help translate the letters into a cohesive narrative. Welch first heard the story while having dinner at the home of a friend, literary agent Sarah Burnes. “I said to her, ‘This is so up my alley,’” says Welch. “I love a heartfelt story about how the smallest thing can make the biggest impact.” Though not considered a YA title, Welch’s first book, the memoir The Kids Are All Right, is a story of teenagers, told from multiple points of view – her own, at age 14, as well as those of her three siblings.

Welch had also spent a summer working in an orphanage outside of Harare, Zimbabwe’s capital city, when she was 21. When she first sat down with Martin at a trendy Brooklyn café, to discuss writing the book, she greeted him with “Mangwanani,” or “Good morning.”

“He almost fell off his seat!” she says. “We talked about the food – how I loved sadza! And how I had hitchhiked around the country that summer for two weeks. I know he was happy that I had visited his country – he felt it was important, and I did too.”

Martin says he was eager to get his story out to young American readers. “I’d like them to know how lucky they are,” he says. “It’s easy to take for granted experiences such having a school bus to pick you up and drop you off from home, but in parts of the world like Africa, there are students who walk to school 10 miles daily on bare feet.”

But the act of telling it was tougher than either he or Welch had imagined. “He was a lock box,” says Welch, who flew to Durham, N.C., to spend the weekend with Martin and to get, as she describes it, “48 hours of deep download.” There was a level of trauma, from years of living in fear and poverty, that made it hard to get into his deepest emotions and the details. “It was up to me to push him,” she says, “and to take his incredibly matter-of-fact, deliberate storytelling and flesh it out.”

The experience was quite different for Caitlin, now 30, who lives outside Philadelphia with her husband and two young daughters. “I had a really good connection with Liz, right from the very beginning,” she says. She too struggled to remember how she felt in the moment, but she had the old letters and her journals, recounting school, friendships and boyfriends, as guides.

As the two women sat together in Caitlin’s childhood bedroom, old scrapbooks and slambooks on their laps and Caitlin’s boyfriends’ names still written on the wall, memories came flooding back. The challenge here, Welch says, was to make Caitlin’s story feel unique and deeply emotive. “People could say ‘white privilege,’ but it’s so much more than that,” she says. “I had to push her – why did you care so deeply? You could have just gone to the movies with your boyfriend. Why didn’t you?”

As Liz wove the stories together, alternating points-of-view between chapters, some details – dates, names – inevitably escaped them. Caitlin’s mother, however, had saved not only Martin’s letters, she’d also saved thousands of photos and even details of the care packages they’d sent back and forth, as well as the Western Union receipts.

“She was my fact-checker, my PI,” says Welch. “Hugely important to this story is the roles the two mothers played.”

Martin’s old letters and documents, still with his mother in Zimbabwe, were harder to come by. But he still traveled back there to care for his family, whom he was supporting. During their second North Carolina meeting, Martin presented Liz with a plastic bag filled with a pile of letters, unorganized and smelling of smoke. “They were all there,” she marvels. “His report cards, his acceptance letter to Villanova... everything.” It was living proof of the power of their connection, Welch says: “Martin’s mother also knew their story would one day be told.”

As complicated as the process was, for Caitlin, it was also deeply grounding, reminding her of how she hopes to expose her own children to a world beyond Hatfield. The book, she believes, speaks to the power of personal connection: “Just because someone might be in a slum in Zimbabwe, it doesn’t mean they’re any different than you,” she says. “We lived thousands of miles apart, but we were so similar.”

For Martin, like Welch, the power of their story is in showing the impact that one person can have on another. “Some efforts that we might deem as minuscule,” he says, “can actually be the lever to change another person’s life.”

I Will Always Write Back: How One Letter Changed Two Lives by Caitlin Alifirenka and Martin Ganda, with Liz Welch. Little, Brown, $18 Apr. ISBN 978-0-316-24131-1