This third edition was the first one to have a real professionnal look, with a solid cover in fake leather. It is also the first one with such a thin background (just a few pages), with the idea from the beginning to purpose a large display of items sold apart.
The concept doesn't differ much from previous editions: you play a group of angels (Magna Veritas) or deamons (In Nomine Satanis) sent back to Earth for some purpose vaguely related to the fight between Evil and Good - and quite often part of sophisticated intrigues between your overlords.
This books mostly contains the core rule system, including lots of powers, skills and curses, and how they are related to the Evil Lord or Archangel you work for. Still, there are lots of little (humoristic) texts that give you a good idea of the atmosphere.
Compared to the previous editions, this one is more centered on forces of Good and Evil, while, say, faeries or vikings disappear. The rule system is simplified to the minimum and a huge attention is given to specific powers and gifts.