The Ghosts of Cougar Island by Peggy Parish, 1986.
This book picks up soon after the previous book in the series ends.
The summer is almost over, and soon, the Roberts children will have to leave Pirate Island. Their grandfather tells them the story of nearby Cougar Island. The members of the Cougar family are all deceased, but the last member of the family said that anyone who trespassed on the island would be haunted by the ghosts of the Cougar family who are buried on the island. Although no one lives there now, some people have seen lights out on the island at midnight. The children’s grandfather even tried to go there once when he was young but turned back when he heard terrible moans. Liza, Bill, and Jed decide that they can’t possibly go home without trying to investigate this haunted island!
Strange things are happening on Pirate Island, too. Someone has been stealing food from Hermit Dan, but someone has also weeded his garden. On the one hand, he appreciates the help, but he can’t understand the thefts.
The children’s grandfather borrows a canoe for them, and the three of them take a trip out to the island. Sure enough, they encounter the terrible moans and a horrifying sound like the rattle of bones. Hermit Dan later offers them an explanation that helps set them at ease, but he can’t explain every mystery. Another visit to the island answers some of the children’s questions and leaves them with others, and a terrifying time in the old abandoned Cougar mansion convinces them of one thing: There is still someone on Cougar Island who shouldn’t be there. Someone who ate the lunch the children left behind earlier. Someone who lights candles at night in the old abandoned Cougar mansion. Someone who may even be responsible for the thefts on Pirate Island.
The book is available to borrow and read for free online through Internet Archive.
The Haunted House by Peggy Parish, 1971.
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.

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.
The Haunting of Grade Three by Grace Maccarone, 1984.
Most of the group members are misfits in one way or another. Norma Hamburger is a shy girl, frequently teased about her last name. Debbie Clark is a talkative girl who’s really into science. Chuck Webber, Adam’s best friend, is the class clown, always telling stupid jokes (and one of the main people who teases Norma about her name). Danny Biddicker is strong and good at sports, but he worries that he’s not as smart as the others. Joey Baker feels overshadowed by his large family, so he tells tall tales to get attention. The other kids don’t understand and get annoyed with his lies. By working together, they not only learn the secret of Blackwell House, but they learn more about each other and become a real team and friends.
The Treasure of Kilvarra by Elizabeth Baldwin Hazelton, 1974.
Without the amulet to protect her, Christie continues her search for the treasure. An accidental injury takes her to the place where it is hidden, but it’s a dangerous place. Caught in a terrible storm, the children explore an ancient stone tower. There, Christie sees the ghost of a long-dead monk, gesturing to her, begging her to follow him to the treasure that she seeks. The monk died protecting it, and it’s a very unexpected but wonderful treasure indeed. But, getting out of the place is going to be even more dangerous than getting in.
The Dastardly Murder of Dirty Pete by Eth Clifford, 1981.
Harry Onetree and the girls find a ghost town with a hotel, an opera house, and several other buildings. Although Harry only means to look around for a little while, he forgets to set his parking brake (something else Mary Rose warns him about, which he ignores), and their car rolls backward into a ditch. Since it’s getting dark, they’re stranded in the ghost town for the night. But, they’re not alone there.
The Ghost that Came Alive by Vic Crume, 1975.
They tell the kids that the house is supposed to be haunted and people can often hear the ghost of Andrea Cliff calling out in the night. The kids soon hear this spooky voice themselves, calling for help. Trapped in the house with the ghost and unfriendly people, the Blairs decide that they have to figure out what’s really going on, but the danger is worse than they know, and Jenny’s premonition of doom is getting stronger all the time . .