This is another one of those books that I remember from my childhood that I just can't place. I read it as a paperback book, probably from Scholastic, somewhere in the late 1980s to the early 1990s, in English. A family moves to a new house. I don't remember the circumstances. There were either one or two kids, but one was a girl. She starts to see strange things out of the corner of her eye, including the ghost of a small boy. She tries to track down who he might be, and discovers that a boy of that description and his father disappeared from the house some years ago.
Ultimately, she comes to realize that the boy's father was a thief of some sort, and that the item stolen was a set of rare stamps, which I think turn out to be inside a picture frame. There's a scene near the end where they're trying to recover the frame while in the middle of a storm, with part of the danger being that the father is also a ghost, and may be able to cause actual harm. But, of course, they do find the stamps, which I think lays both ghosts to rest.
The cover showed the ghost boy, I think in a blue outfit and translucent, in front of a swingset with the protagonist girl also in the foreground. I believe that the boy was reaching up to her with a sad expression on his face. I'm pretty sure that the boy was blond, and was not wearing a hat.
I think that I found this one at least once before a few years ago, but I'm having no luck remembering what the title was.