I've read the entire Masters of the Shadowlands series (to Book 13), but this is my favorite, which I keep coming back to and re-reading with delight.
I'd agree that as a series, the plot of these stories becomes a little repetitive. As this is early in the series, it didn't detract from my enjoyment. I loved the relationship between Gabi and Marcus, and seeing other characters I've come to think of as old friends now through the series was fun. I'd also agree that Gabi is over the top in her bratty behavior, but that's the role she's playing, for a very good reason. Marcus does realize her behavior isn't consistent, and I liked the way he reacted, which was smart and measured and in character.
There are some absolute stand-out scenes in this book: the figging, the pursuit through the Shadowlands, the doggie punishment. They're wonderful to read - the author has such an amazing imagination. There are scenes like this throughout the series, but the ones in this book stand out in my mind because of the warmth of the relationship between Gabi and Marcus, which I didn't always feel in some of the other books.
On the whole, I really enjoyed this series, but this is my favorite and I will definitely keep reading and recommending this author.