One summer about 85 years ago in a small Alabama town, a scrappy tomboy named Nelle met her new next door neighbor, Tru, a bookish, dapper dresser with a high-pitched voice and a mischievous streak.

They made an unlikely pair. She often went barefoot in overalls while he dressed so fastidiously that a teacher said he stood out like a bird of paradise in a flock of crows. But both were oddballs who took refuge in detective novels, and they quickly bonded over their mutual love of Sherlock Holmes and the Rover Boys, spending long afternoons reading mysteries in their treehouse sanctuary. To entertain themselves, they started writing their own stories on her father’s Underwood typewriter, taking turns as one of them narrated while the other typed.

They grew up to be two of the South’s greatest writers — Harper Lee and Truman Capote — and their lives and work were intertwined long after that first summer. Ms. Lee drew on their friendship in her portrait of the characters Scout and Dill in “To Kill a Mockingbird” and in her newly released novel, “Go Set a Watchman.” Mr. Capote based the brash, sharp-tongued tomboy Idabel Thompkins in his debut novel, “Other Voices, Other Rooms,” on Nelle. They worked together on Mr. Capote’s true crime book “In Cold Blood,” then drifted apart after Mr. Capote failed to credit her properly.

Their broken friendship has been restored — in fiction, at least — in a forthcoming middle-grade novel, “Tru & Nelle,” by Greg Neri. Though Ms. Lee and Mr. Capote have each individually been the subject of numerous biographies, documentaries and feature films, “Tru & Nelle” is the first book to focus primarily on their childhood bond.