The Dead Man in Indian Creek by Mary Downing Hahn

When Parker Pettengill and Matt Armentrout find a dead man at the edge of Indian Creek, their camping trip comes to a close -- and a detective hunt begins. Evans, the creepy new boyfriend of Parker's mom, Pam, was seen at -- the place of the crime -- but there's no solid evidence that he committed the murder. Parker is convinced that Pam's boyfriend is to blame, and he will not rest until he has proof. And when Parker and Armentrout hide out in Pam's antique store and spy on Pam and Evans, they find out more than they bargained for....