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.
One Ghost Too Many by Drew Stevenson, 1991.
Sarah persuades Clark and his friend “Frog” Fenniman to join her investigation of the house, but besides the resident ghost, they will also have to deal with a local group interested in psychic phenomena and a mysterious stranger who is paying the local bully to spy on the house.
Jumble Joan by Rose Impey, 1989.
The Flat Man by Rose Impey, 1988.
Scare Yourself to Sleep by Rose Impey, 1988.

The Mystery on October Road by Alison Cragin Herzig and Jane Lawrence Mali, 1991.
The Talking Table Mystery by Georgess McHargue, 1977.

The Ghost on the Hill by Grace Maccarone, 1990.
When they reach the state park, Dennis Ten Foot Bridge, who is the last of his tribe, tells the students about his tribe, teaches them wilderness skills, and leads them in group activities. One evening, Joey goes out to practice some rowing on the lake by himself, and he sees what looks like a large ghost at the top of a hill However, everyone knows that Joey lies about a lot of things, so at first, no one believes him. At least, no one except Adam.
The Return of the Third-Grade Ghosthunters by Grace Maccarone, 1989.
They try to use scientific methods to study the haunting. They measure the placement of the furniture to see if anything moves while they’re out and sprinkle flour on the floor to get footprints. But, even though they watch carefully and don’t see anyone enter the bunk house, when they go inside, everything is a mess, and the flour doesn’t appear disturbed. Everyone else is about ready to admit that this one is a real ghost, but Adam has another theory.