The Twilight Watch - The Night Watch Trilogy (Sergei Lukianenko)

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. Night Watch Agent Anton Gorodetskys holiday is abruptly shortened when an urgent call from Gesar - his boss and Night Watch head - forces him to return to work. Gesar has received an anonymous note, stating that an Other has revealed the full truth about their kind to a human, and intends to convert the human in an Other. The note has also been sent to the Day Watch, and to the Inquisition - but only the very highest-level Others know the addresses. So the Inquisition orders the cooperation of Night and Day Watch in an effort to unmask the culprit...