Toying With Danger by Drew Stevenson, 1993.
Sarah Capshaw is lamenting to her friend Clark that she hasn’t had an interesting case to solve since she and her parents moved to Wilsonburg when their friend Frog tells them that the local bully saw a monster at the old Harley farm outside of town. Rumor has it that a mad scientist bought the place. Naturally, Sarah wants to investigate.
It turns out that the “mad scientist,” Dr. Becker, is actually an eccentric toy inventor. The “monster” is an electronic Frankenstein monster. Dr. Becker is actually pretty nice and is even interested in seeing the detective board game that Frog is trying to invent.
Even though the monster wasn’t a real monster, there are still strange things happening in the woods surrounding the old farm where Dr. Becker’s workshop is. The local bully is hanging around in the woods and trying to scare everyone away. The kids also see a mysterious man hanging around, and Sarah wonders if he could be spying on Dr. Becker in order to steal his designs.
It seems that Sarah is right that someone is trying to spy on Dr. Becker. The kids learn more about the money involved in buying and selling toy designs when they visit the Too Wonderful toy company for a tour with Sarah’s grandfather. Making toys is serious business, and companies guard their designs very carefully. The Too Wonderful toy company wants to purchase some of Dr. Becker’s designs, but one of the members of the company says some strange things about Dr. Becker. Can the kids trust him? Can they trust the strange Dr. Becker? Can Sarah catch the spy before it’s too late?
This is part of the Sarah Capshaw Mysteries series.