On looking through it in a bookshop, I remembered a German film "Der Himmel über Berlin" (1987).
This "After dark" looks like the first half of this film. That is to say, the hero of this novel is observing the world below like an angel, and sometimes sets foot on a room or someting else in order to be closer to somebody.
The following site is also useful to know the plot:
The film director is Wim Wenders, and main actors are Bruno Ganz, Otto Sander and so on.
По-русски "Небо над Берлином"
English title is "Heaven over Berlin" (or "Wings of Desire")