While this take on dragons doesn't completely work for me (if they like burning carbon so much, they why don't they simply cause forest fires everywhere to make their "candy" for themselves?) but I have to admit it's an interesting variant and doesn't feel too contrived. There were parts of the world-building that lost me however, given that having giant dragons regularly attacking your population would change history rather dramatically in some ways. I can see the world turning out very similar to ours pre-industrialization (since clearly that pollution will attract more dragons than anything else) but the fact that it's so similar to ours even after that point, that more or less broke my suspension of disbelief. The story even tries to skirt around it in an odd way, indicating both that people came to the New World to try and avoid dragons instead of religious persecution being one of the early causes but then mentioned that the Old World dragons seemed to come with which left me wondering "what, did some moron pack eggs with them?!" and I'm wondering if I misread that passage, especially since Johnston could have easily said that the New World had even more dragons than expected so it ended up being a moot point for the new settlers.
But most of all, one of the big points in the story is how all dragon-slayers have a tour of duty with the oil watch, a conglomeration of companies (I don't believe it was countries) to protect the world's oil fields, and one of Siobhan's teachers gives them an assignment that's supposed to make her realize how these unfairly leaves smaller industries vulnerable. It didn't seem to have much of an effect on her (possibly since she's already lived in a small town without a dragon slayer for most of her life and she's familiar with it first hand), leading me to wonder why the scene was even kept. Then I couldn't stop thinking about how this story was completely ignoring China's industrialization (which would be a feast for dragons) as if it never happened but yet China must still be a major producer in this alternate reality judging by the level of ready-made and cheap consumer products in the characters' lives! Johnston was trying to be clever with her idea here but either a lot of exposition was cut from this book or she didn't go deep enough into the world building to clear up a few details. It's my biggest problem of the book easily and, while it's not one that will keep me from recommending it to people, it's an itch I can't ignore.
Where the story falls short in the world-building it almost makes up for it with the characters but instead of talking about Owen and Siobhan I want to start with another character first. Siobhan has a classmate who she's not particularly close to at the beginning of the book who makes an attempt to become closer friends. We the readers, and Siobhan, know that she's interested in Owen and wants to know more about him and hopes that getting closer to Siobhan will also help, yet this character is never written as a shallow-minded, teenaged girl but is instead allowed to develop the way a real person would. We see her and Siobhan make cautious steps towards each other and become real friends with a close bond that's built on something other than Owen and later the girl admits that while she still wants to get closer to him it's no longer for romance, she also wants to be a dragon slayer and knows that he's her best chance to try and do this. That's development that usually only a major character would get in a story, that kind of complicated, growing relationship, and I ended up liking her just a bit more than Owen and Siobhan. Both of them are great characters as well, the natural, not awkward but not smooth way they become closer classmates, out of convenience, and then friends, from how that convenience turned into actual choices, was wonderful and felt like so many friendships I had in high school and college. While I'm not sure what still needs to be covered in a second book, all of the themes presented here were either wrapped up neatly or clearly ideas that will take years if not decades for Owen and others to implement, I would love to read more about all of these characters so I will be sure to keep an eye out for a sequel!