This is the first book of the series (of six books).

Excerpts from the book:
...The streets of Mokoshin, which were mainly built up with multi-story standard buildings made of gray panels, were illuminated by the spring April sun. Or as the people called its 'Beautiful Golden Ra'. It touched with its golden rays the foliage of trees, green grass, buildings, roads and playgrounds.
In one of the houses, on the outskirts of the capital, in a girl's room with soft pink painted walls, an alarm clock rang. The owner of the room, a sixteen-year-old girl Zima with identification number X-0-2212938-R0101, reluctantly woke up. At night she had a bad dream: that her older sister Vesna was falling into an abyss...
***
...Zima moved closer to the oven and opened the book. She saw the usual illustrations. "I dreamed that I turned into the White Fox because I loved looking at illustrations for this fairy tale as a child?" the girl involuntarily thought, remembering her strange dream that she saw on that tragic day. Suddenly she realized that letters began to appear on the illustrations...
The letters formed words written in her sister's familiar handwriting. Zima widened her eyes for a moment. "Ink that becomes visible when heated?" she understood. "Vesna once told me that when she was looking for material for an article, she read about it in some book from the library archives... Did she really make it herself? And what did she write here?"
The girl felt so much amazement that she did not cry, despite the bitter memories of her sister. And she opened the book on the first page and began to read. And she read the following: "Dear Zima! If you are reading these words, then most likely I am no longer alive. I don't know how I will leave this world, but it will probably be in such a way that I myself won't understand what's wrong when I start to die..."
***
...She approached the cat, but he suddenly ran away.
"Where are you going?" the girl exclaimed in surprise.
"Ms. Avril, if you don't hurry up, you'll be late for school!" Mr. Claude called her.
"I know, but there's a cat here!" she answered.
"Wow! This is true! Have your neighbors got a second pet?" the man was surprised.
"I don't know, but he can't seem to get over the fence," Avril sighed.
She approached the cat. This time he stood calmly in place.
"I want to help you! Why are you, stupid, running away from me?" the girl asked displeasedly, taking the furball in her arms.
Of course, she didn't expect to hear an answer. But suddenly, the cat, as if understanding every word she said, pointed his paw to the side. And the next moment, surprised Avril realized that she was standing in the middle of the fog.
"What?" she said in surprise. "Where did such a strong fog suddenly come from?"
The girl looked around, but saw nothing: everything around was covered with a thick white veil.
"Mom? Mr. Claude?" she called her mother and the gardener-driver. "Where are you?"
Nobody answered her.
"Mom? Mr. Claude?" Ms. Edelweiss was seriously frightened.
"Meow!" the cat meowed.
He again pointed his paw to the side. Avril looked there and suddenly realized that the fog had parted and she was in an unfamiliar place.
Avril discovered with considerable surprise that she was not standing in the garden near her house, but in a hall with high arches and walls of white-silver stone. Columns supported the vaults, and on the ceiling there was an intricate painting depicting magical formulas that the girl had never seen before. And in front, in the center of the hall, stood a silver throne.
It was a simple throne, without unnecessary decorations. One might even say that it looked very ascetic.
"Where am I?" the girl asked in surprise.
Suddenly a voice was heard. It seemed to come from everywhere..."