To start with, I didn't realise that this was a prequel until about a third of the way through. While it has a plot that is resolved at the end of the story, it is geared towards setting things up for the next book. I would recommend looking at this story to see whether the writing style suits you but considering reading the rest of the books first and then using this to delve more into the characters if you're interested.
Based on the writing style, the target audience for this story appears to be 10-14ish years of age.
I think it would have benefited from being a little longer so that there was more opportunity to expand on things, although that might be in part a byproduct of the age range it's going for.
I am a little concerned about how what looks like it will be the romantic interest will go - it appears to be setting up a 'they dislike one another, but is that all there is?' type situation. From the preview of the following book, the dynamic doesn't look problematic/unhealthy, but those kind of dynamics are often either handled very well or very poorly. Hopefully this is the former, but just a heads up.
I am curious about Amery's project though. I thought the initial reveal could've been more subtle so that it built to the realisation of her secret, but by the end I was curious about what the project entailed and what was going to happen in the next book. Not curious enough for me to buy it at this point - I'm super-busy and only chose to read this one because it was short and looked kind of interesting - but curious enough that I haven't ruled out checking it out in the holidays. [spoiler](I don't really think it's a spoiler, but in case) I don't see why she can't pretend that the mark is a tattoo if she is the age I think she is - 17-18 - though. I can get not wanting people to know because, hey, the less people know the better and you never know who they'll mention it to, but if you've been keeping it hidden for that long you'd have probably also created a cover story in case someone did accidentally see it sometime. [/spoiler]