The Brown Fox Mystery — Ellery Queen Jr.

During a lakeside vacation, Djuna hunts a mysterious fire starter

Whenever Djuna leaves the house, he seems to get into trouble. Whether it's catching bank robbers or sniffing out a ring of counterfeiters, something about this young man makes him a magnet for mystery. When Miss Annie Ellery plans a summer vacation by the shores of Silver Lake, Djuna and his friend Tommy while away their time fishing, swimming, and sleeping till noon. Djuna promises to stay out of trouble -- but it isn't long before trouble finds him.
 
Djuna's best friend at Silver Lake is Captain Ben, a fisherman who shows him the tricks of the trade. When Ben's boat and fishing shack go up in smoke, Djuna promises to find the person responsible, even if it means getting into more trouble than ever before.
 
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 Brown Fox Mystery is the fifth book in the Ellery Queen Jr. Mystery Stories, but you may enjoy reading the series in any order.