The Dragon in the Ghetto Caper — E.L. Konigsburg

Andy's not your average resident of exclusive Foxmeadow -- whenever he sits down to draw something, it turns out to be a dragon. And he wants to be a detective when he grows up -- not just an ordinary, everyday police detective, but a tough, cool, famous detective like the ones he reads about in mystery novels.
Everyone knows a famous detective needs a sidekick, but Mrs. Edie Yakots, a lonely new bride who's just moved into Foxmeadow, isn't exactly what Andy had in mind -- he sometimes has a hard time just figuring out what's she's talking about. But she's the only volunteer for the job, and before he knows it, she's led him right into the middle of his first real crime -- in an inner-city neighborhood a short drive, and light years away, from Foxmeadow.