I am looking for the title or author of a book I remember reading as a child in '70s or '80s.
I don't remember much about the plot. Most of the book played in our world, but there was a door or portal where something or someone from another world came through, resulting in danger to the protagonists or their families.
The protagonists were children, as is the case in many children books. However, I remember the book having a rather dark and somber mood, unusual for a children's book.
One distinct event in this book I remember: someone went missing, and all they found was their glove, which was embedded in a rock (or in the ground), with part of it sticking out.
I think there also was a unicorn, no the tame, fluffy rainbow-creature currently so popular, but a wild and dangerous beast.
I read the book in german, but I don't know whether if was originally written in german, or was translated from another language.
The book was a hardcover, and while I can't remember the design of the sleeve, I remember it being mostly orange or red in color.