I happened to pick up a copy of this book in a garage sale and am amazed it isn't included in children's reading lists; esp for middle schools where kids need to deal with their other selves rather than their other selves dealing them.
Summer Job starts out with Virgil in conflict with Cy, the voice he hears in his head. He fears that if things go on as they are Cy will take control of his mind. Forunately, along comes an opportunity to get away to a different environment while the getting is good, i.e., before Cy takes over and won't allow it.
In his new habitat, Virgil is better able to deal with Cy's sarcastic and troublesome nature as they, together, while in the course of solving the misterious disappearance of Virgil's workmate, become involved in uncovering multiple murders that lead to a suicide.
In the course of his summer job Virgil finds that being a loner is not so great after all; as he not only learns the value of friendship; but as well, develops a crush on a fiesty tomboy whom he must leave behind and return to city-fied life: this time though, with a more constructive relationship with his other self.