Este é o primeiro livro da excelente série Night Watch de Sergei Lukyanenko.
De Wikipedia (somente em partes relevantes; estresse meu):
In the story's worldline there exists a magical realm beneath the surface of all things — referred to as the Twilight. The Others were the humans from long ago who figured out how to step into the Twilight. The aura of any Other, or emotional state at the time of their first entry into the Twilight, determines whether or not the Other will become a 'Light' or 'Dark' Other.
[...]
A reluctant mage recently reassigned to field work, Anton Gorodetsky tracks vampires by drinking blood and channeling them. Anton finds the vampires who have been calling Egor with their power and, as they don't have a special license, kills one of them, while the other (a female) gets away.
He returns to the Night Watch headquarters, where his boss, Boris Ignatievich, informs him that he could be in danger as Zabulon (head of the Day Watch) might want revenge for his actions in killing Dark Others and gives him a stuffed owl called Olga (who later transforms into a woman), for his protection. He then discovers that Olga can speak and is a sorceress trapped in an owl's body as a punishment.
Mais tarde é revelado que
Olga is "the boss's lover from way back" (quote).
É uma série de livros fascinante e vale a pena ler.