I read this sometime in the mid-80s. I don't think it was more than 100 pages or so.
A boy somehow comes in contact with an Artificial Intelligence who lives in a larger mainframe computer, I think via dialing in from a computer at home. The AI shows him how to modify a pocket calculator so they can communicate while the kid travels around his town. The boy lives in or near a farm town. He discovers that a big company owned factory orchard is poisoning the town's water supply with some kind of insecticide run-off, and the Artificial intelligence helps solve this mystery and uncover the crooks.