Mystery of the Vanishing Forest — Gertrude Chandler Warner

The Aldens travel to Sumatra to learn about saving the rain forest and its animals. Henry, Jessie, Violet, and Benny are off to the rain forests of Sumatra, where a protected piece of land is home to endangered animals such as orangutans, elephants, and tigers. But when part of the forest gets cut down, the Aldens have to figure out who is responsible, not only to protect the rain forest, but also the animals within.