This book focuses mostly on Gin's past, really showing what made her the person she is in the series. We've seen glimpses of the girl she was, both before Fletcher took her in and during her training as the Spider. In this book, we get to see one of the first (the first?) cases that she took on as the Spider.
I had expected this book to be a real prequel, all in Gin's POV just set in the past but this book has parts at the start and at the end that are in the present. This book is all about Gin telling Owen about an assassin job she'd had when she was 20. I can't think of a lot of things that I can really say about the job that wouldn't be major spoilers, but I will say that Gin and Owen have meet at least twice before they met in the series (the first time Gin gave him food when they were teens and once in this book where he gives her a ride).
He also asked her out during the story Gin tells and I wonder how different Gin might have been if she's said yes: he's the only guy that's ever really been okay with who she is (I'm going to forgive him for being an ass in the past few books, since that was explained).
I wasn't really that surprised by anything that happened during the job. There are clues that seemed a little too obvious to me, maybe they aren't to most or something but I kind of expected it to go the way it did and I would have been surprised if it hadn't. There, that was cryptic right?