Demonology by Volkov. Collectors of Souls

Alice Grechishnaya learned from childhood that in this life you can only rely on yourself. Right after school, she had to go to work to help her mother and younger sisters, and her main problem still remains finding money. It was the promise of a good salary that brought her to the house of a dark sorcerer for the position of personal assistant.
Leon Volkov is that sorcerer. Many years ago, he nearly died and, in order to survive, got involved with forces that are better left alone. Leon has access to the most dangerous secrets of this world, and he needs an assistant like Alice: brave, stubborn, desperate, willing to do anything for money. Because what she is about to face could scare even the bravest people.