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The Kingston Castle by Ruth Benjamin, a Jewish-themed (NB: the author is an Orthodox Jew according to her bio-blurb and many of her books are generally filed under Religious Fiction in the Amazon categories) YA mystery novel originally out from specialty small press Hachai Distributions in 1991 under the pseudonym of Ruth Abrahamson.
When the Rosenbergs decide to vacation in Kingston, it is to get away from the bustle of city life. Little do they know that the town and its famous castle hold in store the adventure of a lifetime.
The Rosenberg kids become fast friends with the Sandler family. The Sandlers are Orthodox Jews and the Rosenbergs begin to discover a world about which they know very little.
Meanwhile the children become involved in a terrifying mystery.
Quinton Holiday Adventure, another YA mystery, which the blurb says was originally published in serial format in Concord Magazine.
The Dinansky Family have gone to spend their holiday in a caravan park in the seaside resort of Quinton. They have taken with them two old transmitter sets (walkie-talkies) from the Second World War which their grandfather has lent them.
Walking along the beach, at some distance from one another,the two older children are talking to one another on the transmitters. Chaya trips and and the dials move frequency and she hears from the machine a man's voice, a voice full of terror. "Please, please help! If anyone can hear me, please help".
Immediately they are all plunged into a breathtaking and dangerous adventure.
Benjamin also offers a few more self-published-looking YA things and some adult-level backlist literary & mysterious identity thrillers, if you're interested.