The Boggart, by Susan Cooper (pub. 1993)
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I read this in 2020; before finding it in a used book store, I don't know if I'd ever even heard of it. It's basically a kids' book, though there was some language in a few spots that I was a bit surprised to find in a book aimed at kids. And... I thought it was an okay story, but I wasn't particularly into it. Maybe I would have liked it more if I'd read it as a kid, or maybe not. (Though actually, I was a bit beyond the target age group by the time it was published.)

A family from Toronto, the Volniks, inherit a castle in Scotland. The parents are Maggie (who runs an antiques shop) and Robert (who runs a local playhouse), and their kids are 12-year-old Emily and her 10-year-old brother, Jessup. The four of them go to Scotland to inspect the castle, and they meet a local boy named Tommy, who is aware that the castle's recently deceased owner, Devon MacDevon, had shared the place with an invisible, mischievous spirit called a boggart. But he doesn't tell anyone about that. The boggart winds up getting trapped in a piece of furniture that Maggie has shipped back to Toronto, for Emily's bedroom. When they all return home, lots of strange things start happening, for which the children get blamed. They eventually learn of the existence of the boggart from a couple of the people of Scottish heritage who work at the playhouse. Meanwhile, there's a psychiatrist named Dr. Stigmore, whom the kids call "the creep," who believes the strange occurrences are the results of psychokinetic energy generated by Emily, and he wants to have her admitted to a hospital to study the phenomenon. (Incidentally, when Emily first started calling him the creep, it was because she was upset that he had bought a piece of furniture from her mother's shop, which she had wanted for herself. At that point, I didn't think she was being at all fair to him. But obviously, later in the story he does become a real creep.)

Anyway, we get to see things from the kids' point of view as well as that of the boggart, which doesn't understand the new world in which he finds himself, but he soon finds new ways to have fun making new kinds of mischief. And his perspective is certainly different from that of the children; he doesn't mean to cause them trouble, either with their parents or Dr. Stigmore, but he can't really help it. Meanwhile, Jessup has a few friends with whom he's designing a computer game about black holes, and that eventually ties into the main plot in an important way that I don't want to spoil. And I guess that's all I want to say about the book, except that it does have a happy ending.

There are a couple of sequels, which I wouldn't mind reading, but it's unlikely that I ever will.