And All Between — Zilpha Keatley Snyder

Eight-year-old Teera finds herself above the powerful Root that keeps her Erdling community captive underground. The Erdlings are feared and despised by the Kindar people who live in the massive trees above. Raamo and Neric, two young Kindar discover Teera and for reasons of their own decide to keep her hidden, allowing her to live with Raamo's parents and his sister, Pomma. But Teera's existence does not remain a secret and soon she, Raamo, Neric, and their friends are all in danger. Published by Atheneum originally in 1979, And All Between is the second book of the Green-Sky Trilogy.