This book focuses exclusively on the races of the Abaddon (aka Obsidian Twilight) and not the actual campaign/realm. For the actual races and their descriptions I would highly recommend reading Endzeitgeists review as it gives great descriptions of the races.
One of the first things one will notice, and Endzeitgeist mentions it, is that the new races seem overall stong. While this may seem problematic it actually fits in pretty well to the living conditions of Abaddon (the book does describe the realm to some extent through the races history). This realm is the living embodiment of "Survival of the Fittest". An example of this is darkvision. It is greatly extended from the normal 60 feet range to 120 feet, once more this is because of the realm is in the Twilight where it seems that everything wants to kill you.
One of the more unique aspects of these races is that many have diverse backgrounds regarding character creation. The Osirians, for example, can be slaves, free birth, or Vault-born. Each option has its own perks and "Faults"(if your into true role-playing).
As for the actual book itself this is one of the better third party books that I have read. I did not notice any gramatical errors and the artwork ranges from okay (core races) to excellent (new races).
If you want to have a dark/gritty PC I would recommend this book regarding the races as they will work nicely. If you want to make an actual Abaddon/Obsidian Twilight Campaign, I would recommend getting the actual campaign book as it contains the races, in addition to the actual background of Abaddon.