"Goosebumps, Welcome to Camp Nightmare" by R. L. Stine

Next summer you'll stay home...if you survive! The food isn't great. The counselors are a little strange. And the camp director seems demented. Billy can handle all that. But then strange things start to happen after dark; his parents won't answer his letters, and his fellow campers start to disappear. What's going on? Camp Nightmoon is turning into Camp Nightmare! And Billy might be next.