The Mystery of the Night Explorers, by Mary Adrian (epub/Kindle/pdf)

City kid Randy is introduced to the wonders of nature by his country cousin, Jimmy, and eventually begins to love life in the country. The two boys build Hidden Lookout, a treehouse, and begin keeping watch on night animals. During the day, they follow clues to try to discover a long-hidden treasure.

"A quietly pleasant summer story set around the routine treasure hunt provides as well the unusual habits of common country 'critters.'" -- Kirkus