The Amazon description of the book captures this well: dark and comic. Some of the scenes in the book were so funny that my laughter frequently drew too much attention of my fellow early morning commuters. However, the book also has an ever-present dark undertone, and one cannot help feeling slightly disturbed about the night-and-day actions and thoughts of the main character. Although it is difficult to attract sympathy for him, you sometimes can't help him wanting to come out on top.
The description of Reykjavík and its surroundings are excellently portrayed, albeit cynically by the main character. The rougher edges, as well as the beauty, of this wonderful city are captured to a tee by Helgason. These will especially ring true with anyone who has visited Iceland or Reykjavík.
Read the book and watch the film; they are very different and I much preferred the book over the movie as they develop the main character, and some of the peripheral ones, a lot more.