Shadow-Catcher by Betty Levin

It is 1892, and Jonathan Capewell, a farm boy who dreams of becoming a big-city detective, is sent from home to look after his mysterious grandfather. Grandpa is a traveling photographer, and his independent ways have never included family members -- certainly not his youngest grandchild.

After a grueling journey, Jonathan and Grandpa shoot an image of a puzzling struggle on a raging river in the Maine woods. At first they don't suspect it's anything more than a logging accident. But later the scene comes back to haunt them when a stranger shows an uncommon interest in the undeveloped negatives.

Who is this over-friendly stranger? Why does he seem so determined to have those pictures? The clues point to something that Jonathan has already begun to suspect: what happened on the rapids that day was no accident....