I'm not sure specifically as it's not a widely known common title. It could be a fan - made or very niche 'Star Trek: The Next Generation (TNG)' related novel with a 'dark mirror' concept, perhaps exploring alternate, darker versions of the TNG universe.
The 'st tng dark mirror novel' might be a unique take on the 'Star Trek: The Next Generation' universe. The 'dark mirror' idea could be a reference to the concept of mirror universes that are often explored in science - fiction. In this novel, it could be that the characters we love from TNG are faced with challenges that force them to confront their own dark sides. It might also show a different political or social structure within the Star Trek universe that is far more sinister than the one we are accustomed to in the regular series. There could be new villains or threats that are born out of this darker version of the TNG world.
Well, without more context, it's hard to say exactly. But if it's related to 'Star Trek: TNG', the 'dark mirror' might imply a story that shows the darker side of the characters or the Federation we know from the series. It could involve evil counterparts or situations that are the opposite of the usual positive and utopian view in TNG. Maybe it delves into moral gray areas that the normal TNG storylines don't often touch.