Fantasy story about a knife that can cut holes to other dimensions [duplicate]

Question

I read this story a few years ago (estimate: 8-10 years), so I was around 13-15 at that time. The language of the book was German, but I'm not sure whether or not that is the original language of the book. If I remember correctly, the story consisted of three books in total.

The story starts in some town where a boy either gets thrown out of the house or leaves voluntary. He then wanders around, talks to some neighbor until he comes to some sort of intersection / street with some trees (may have been palms). There a strange phenomenon catches his eye: between two of the trees the world looks somewhat strange, like there is a window cut out of it. Curious, the boy goes through the window and finds himself in another world. There he is being attacked by (ghosts?) and flees into a tall tower, where he finds a knife with a blade so thin you can't see it with the naked eye. It has two sides, which are different from each other. With the knife he somehow escapes / defeats some of the "ghosts" and travels on.

Later in the story, he learns by cutting with his knife he creates those ghosts or kills someone somewhere by doing so. Knowing that, he tries to use it only as much as needed.

Some time later he then opens a window to the world of death where he has something to accomplish (probably rescuing someone).

Answer 1

This is The Subtle Knife, book 2 of Phillip Pullman's His Dark Materials Trilogy. Published in 1997 in English and 1999 in German

Will is twelve years old and he's just killed a man. Now he's on his own, on the run, determined to discover the truth about his father disappearance.

Then Will steps through a window in the air into another world, and finds himself with a companion - a strange, savage little girl called Lyra. Like Will, she has a mission which she intends to carry out at all costs.

But the world of Cittàgazze is a strange and unsettling place. Deadly, soul-eating Spectres stalk in its streets, while high above, the wingbeats of distant angels sound against the sky. And in the mysterious Torre degli Angeli lurks Cittàgazze's most important secret - an object which people from many worlds would kill to posses.

The Subtle Knife opens in a world very much like our own. Will lives alone with his mother. Men have been coming to the house looking for letters that Will’s father wrote to Will’s mother, and Will wants to find them before the men do. Just before Will finds the letters, the men arrive at the house. Will escape with the letters, accidentally killing one of the men along the way.

He then falls through a window into another world, where he fights with the owner of "The Subtle Knife" losing two of his fingers in the process. The Knife allows the wielder to cut windows into parallel worlds, rather than other dimensions. The ghosts, or Spectres as they are called in the English version, are only dangerous to adults rather than children.

The story concludes in The Amber Spyglass where it is revealed that;

A Spectre is created every time a window to another world is opened by the Knife.