I picked this up on a whim at the library, not knowing too much about the Sinestro Corp. This book was a great introduction to them. As we follow a new member of the corp, we find out the stories of how different characters decided to join the Sinestro Corp and why, exactly, they want to destroy the Green Lanterns. I haven't read the Sinestro Corp War yet (which is what these stories were originally part of as side stories and one-shots), but I suspect I'll have a much stronger grounding in it when I find it.
Some of the stories were down right frightening, having me looking over my shoulder. Particularly the story of Despotellis, who literally gets inside someone to kill. It made me shudder in that, "Wow. This is good!" kind of way.
Being backstories as they are, there isn't a whole lot of plot to it. Well, not in and of itself. It does give you a glimpse into the Sinestro Corp War, but it's not a major part of it. I would actually suggest reading this before jumping into the Sinestro Corp War series, especially if you aren't very familiar with Green Lantern history. It should help bring you up to speed on quite a few things. I know it did for me.