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.
Cousins in the Castle by Barbara Brooks Wallace, 1996.
Ghosts in the Gallery by Barbara Brooks Wallace, 2000.
Peppermints in the Parlor by Barbara Brooks Wallace, 1980.
The Great and Terrible Quest by Margaret Lovett, 1970.
#1 The Secret of the Mansion by Julie Campbell, 1948.
The Mystery in Old Quebec by Mary C. Jane, 1955.
The solution to the mystery involves family quarrels, custody issues, and racial tensions. In spite of that, this is actually a very gentle story. In the end, the kids are dependent on the people the mysterious message was intended for to help a troubled, lonely child. It turns out that the boy is an orphan, and some of his relatives are white, and some are First Nations (Native American). There is a fight for the boy’s custody, but the boy knows where he really wants to be.
Just Tell Me When We’re Dead! by Eth Clifford, 1983.
The first place he goes is to an island in the middle of the lake near his house. The island has campgrounds and an amusement park, which is now closed for the season. Mary Rose and Jo-Beth, realizing where Jeff has gone, follow him there. But, the children are not alone on the island. When Jeff is captured by two criminals who are looking for loot that they stashed on the island years before, he has to keep his wits about him to find a way to summon help. Meanwhile, Mary Rose and Jo-Beth have no idea what they’ve just walked into.