As an individual goes into lower levels of the Twilight, aspects of the 'normal world' slowly dissapear and loose color. Each lower level of the twilight requires more effort and energy to enter into. The Others can travel to different levels of the Twilight in direct proportion to their level of power.

The Twilight: Parallell worlds that overlap the existing world in which normal humans live.

Mages, Vampires, Shape-shifters, Forces of Light and Dark: Light Others and Dark Others, Shorts, Anthologies and Guides World Building Setting

The Twilight Watch (2007) aka Dusk Watch.

However, when he discovers a girl under a terrible curse, his mission is diverted: a curse this monstrous means all of Moscow is in danger, and the Night Watch must race to save the city. He has been a sysadmin for the past few years, and this is his first active field mission: to find and stop a rogue vampire. Anton is a member of the Night Watch, the Light's police force against the powers of Darkness. Vampires can hunt and kill only via the approved lottery, and Light mages cannot perform healings and 'remoralizations' at random. Now that they know, there is no alternative to a magical duel, a battle that neither of them wants to win.After eons of war between Light and Dark, the two forces have created an unholy truce: to maintain a balance between good and evil. Suddenly Alice remembers him as one of those involved in the battle that left her crippled.

But then comes a shattering realisation: Igor is a Light Mage. There she meets Igor the chemistry between them is instant and irresistible. Drained of her powers, she is sent to recuperate at a youth camp near the Black Sea. A fierce battle ensues, during which Alice almost dies. But when they arrive, the Night Watch team has already made the arrest.

The team is on a mission to apprehend an uninitiated Other, a practicing Dark witch who has so far eluded the bureaux responsible for finding and initiating unlicensed practitioners of magic. In The Day Watch, second book of the Night Watch trilogy, Alice, a young but powerful Dark Other, attends a planning meeting with her comrades in the Day Watch. 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. Walking the streets of Moscow, indistinguishable from the rest of its population, are The Others.

Sergei lukyanenko books in order