SOLVED - boy haunted by something after picking up a coin

[Marisa]

I was hoping you would help me remember a book I read about six years ago. I read it at my grandparents' house, so it would have been published at least in the 90's or before. It was quite scary and I hated it, but I really want to remember the title. The book was a blue hardcover; I can't remember if it had a dust jacket or not.
A boy is haunted by something in it...I don't know what. I think it was black. I remember that it either snuck into the house through the mail slot in the door or just flapped it. I think the haunting started when the boy picked up something, a coin maybe?
I also think there was a weird symbol on the coin: a upside-down triangle is what I remember.
Sorry this is so muddled! Hope someone can help me find it!

[Tab]

The Figure in the Shadows , maybe?

[Marisa]

YES!! Thank you thank you!!! This has been bothering me for a while!