"Telephone of the Tree" is not, technically, a mystery. But there is a human mystery at the core of Alison McGhee's book, and young readers' ability to "solve" it will be a big part of its warm, comforting appeal.

Tree-loving 10-year-old Ayla is the main character of the middle-grade book, which is set in a neighborhood where the mulberries, crabapples, maples and oaks are sort of a playground/museum combo for Ayla and best friend Kiri. When they were 7-year-olds, both children expressed the wish that they could become trees when they grew up, although they now know that's not in the cards.

In the book's present tense, Kiri has left the neighborhood for unspecified reasons that narrator Ayla prefers not to talk about. So that's one mystery. Another is the (unconnected) telephone that suddenly appears in the branches of Ayla's favorite birch tree and that people use to commune with lost loved ones they have no other way to contact.

McGhee's tender book was inspired by a "wind phone" created by a Japanese man named Itaru Sasaki, who installed an unconnected phone booth in his garden, making "calls" to soothe his grief over the death of his cousin. Now, there are hundreds around the world, including the one in Ayla's fictitious neighborhood.

Eventually, Ayla figures things out. No spoilers here but she gets helps from the advice of a teacher who tells students working on art projects, "The negative space is as important as the positive space. Always remember that."

Telephone of the Tree

By: Alison McGhee.

Publisher: Rocky Pond, 196 pages, $17.99.