This book is what it is: a young adult novel.
That said, it's a very good one. You can read the summary on the book's page, so I won't go into that here.
I loved the juxtaposition of Victorian England, colonial India, and the fairy world. The protagonist doesn't belong in any of them, and she recognizes that, which sets up the whole story: the outsider tries to find her niche.
I didn't care for any of the other main characters, mostly because I felt that the protagonist, Gemma, was treading on thin ice by being friends with them. I liked Gemma - I've read reviews that said she was selfish, angry, and petty, and she can be - but what sixteen-year-old isn't? The friendship between Gemma and the other three girls is based on a desire for freedom as well as the tenuous sharing of secrets - I don't think the girls were ever meant to appear as the best of friends, even on a good day, so the reviews that criticize the friendship being shallow puzzle me. (I mean...well, yeah, right?)
I thought the story flowed really well and had enough twists and turns to keep me guessing - it's also a really quick read and I was sorry when it ended as soon as it did. The visits to the fairy realm were really a delight to read - pure escapism for the characters as well as the reader. And not without a dark edge.
Finally, it was a little racy, which I thought was pretty awesome for a YA novel. It's hard to write a teenage sexual awakening while so much other stuff is going on, especially without being sordid, cheesy, or flowery. Bray does this really well - and the male love interest is your typical aloof, charming, vaguely dangerous, devastatingly hot, man-of-few-words character. I can hear the swoons of teenage girls everywhere. Hell, even I sighed once or twice.
I'm definitely looking forward to reading the next two books in this series.