The Mystery of the Mixed-up Zoo — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they live with their grandfather, who takes them to visit his friend, Edward, who owns a zoo.

The zoo is wonderful -- it has seals and giraffes and elephants and tigers, and Benny's favorite, a little monkey named Amos. It also has a mystery. Someone is mixing up signs, switching off electricity, and endangering the animals' lives. If the Boxcar Children want to save the animals, they'll have to solve the mystery--and fast!