The Mystery in Old Quebec by Mary C. Jane, 1955.
Mark and Kerry, a brother and sister, are visiting Quebec with their father, who is on a business trip. Their father allows them to explore the city on their own during the day while he works (something that would be unlikely to happen in modern times). The kids enjoy sight-seeing and learning French words and phrases. They even make a new friend in the city, but they are troubled by events at the quaint little inn where they are staying.
On their first day there, Kerry’s coat mysteriously disappears and reappears with some strange messages in the pocket. A boy wants them to take a message to someone in the city. At night, Kerry hears someone crying in the next room. Is the boy being held prisoner at the inn? If so, why? Kerry and Mark are determined to help in any way they can, but time is running out.
The book is currently available online through Internet Archive.
My Reaction and Spoilers
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.
The author describes the atmosphere of Quebec in some detail. The famous landmarks probably haven’t changed much, although I don’t know if all the descriptions are accurate anymore. I also thought it was interesting how the author includes some French phrases and their pronunciations and translations as the kids learn to communicate with French-speaking people in the city.
The Blue-Nosed Witch by Margaret Embry, 1956.
Blanche looks for her fellow witches but ends up joining a group of trick-or-treaters by mistake. They love her blue nose and introduce her to the idea of trick-or-treating. Thinking that even the grumpiest man in town would be impressed by Blanche’s amazing nose, they stop at his house, too. The old man isn’t impressed by anything and plays a mean trick on the kids. However, Blanche is a real witch, and she and her cat Brockett give the old man a real Halloween scare.
Terror on Cemetery Hill by Drew Stevenson, 1996.
Toying With Danger by Drew Stevenson, 1993.
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?
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.