The Green Turtle Mystery — Ellery Queen Jr.

To rescue a friend's pet turtle, Djuna must solve the riddle of a haunted house

His new shoeshine box under his arm, boy-sleuth Djuna is looking for customers when he meets a young reporter named Socker Furlong. Socker assigns Djuna and copyboy Ben Franklin to investigate a haunted empty house. Djuna, Ben, and Ben's pet turtle are having a lot of fun creeping around outside -- until the place turns out not to be so abandoned after all.
 
Djuna sees people inside the house, and when he knocks, a young girl answers the door -- only to slam it in his face. Djuna and Ben run for help and are halfway down the block when Ben notices that his turtle is missing. To get him back, they will have to crack the mystery of the haunted house, no matter what ghosts may stand in their way.
 
Ellery Queen is one of the world's finest detectives, but his adventures are nothing compared to the Ellery Queen Jr. Mystery Stories. Join Queen's apprentice, Djuna, and his trusty Scottie, Champ, on adventures filled with danger, suspense, and thrills.
 
The Green Turtle Mystery is the third book in the Ellery Queen Jr. Mystery Stories, but you may enjoy reading the series in any order.