S Lukyanenko: The Twilight Watch [2007]

Walking the streets of Moscow, indistinguishable from the rest of its population, are the Others. Possessors of supernatural powers and capable of entering the Twilight, a shadowy world that exists in parallel to our own, each owes allegiance either to the Dark or the Light. Three years have passed since the events of The Day Watch. Svetlana has left the Night Watch to raise her and Anton's daughter Nadya, now a precocious two-year-old. With mother and daughter spending the summer on a dacha not far from Moscow, Anton is working on uneventfully, dreaming of a holiday, when his boss Gesar asks him in for a private meeting. Gesar has received an anonymous note, stating that an Other has revealed the full truth about their kind to a human, and now intends to do the impossible: convert that human into an Other. Even more worryingly, the note has been sent to Zebulon and to the Inquisition's offices in Berne - and only the highest-level mages and sorcerers know the address. So the Inquisition has ordered the Night Watch to cooperate with the Day Watch and unmask the culprit.
Anton will be the Night Watch representative, while the Day Watch is sending Kostya Saushkin, once Anton's teenage neighbour and idealistic friend, now a High Vampire and, at the age of twenty, the youngest in Europe...