I read the the Night Angel trilogy because I liked Brent Weeks' other major work, the Lightbringer Series. Broadly speaking, Night Angel is about redemption from the shadows. The big themes in the book are love, the ethics of kill-to-save, and the ethics of power.
The book has most of the fantasy tropes that you would expect. There are the old prophecies, old magic that has been lost to the world, and even mithril ("mistarille"). No Elves or Dwarves, though.
Weeks wrote Night Angel before Lightrbinger, and his writing style has certainly improved over time. One of the main things that I like about Lightbringer is that the point of view always closely follows one character at a time, and it matches that character's thoughts, humor, and overall feel. In Night Angel, on the other hand, the point of view partly follows a character, but it partly is omnicient, and it moves from character to character very quickly. As such, it doesn't have the same embodied feel as Lightbringer, and it seems much less polished.
The books aren't particularly novel, but the trilogy and the associated short stories are fairly engaging and moderately well written, and I would recommend them to anyone who enjoyed Lightbringer.