I'd say that Of Fire and Stars is nothing groundbreaking, except that, of course, an otherwise generic YA fantasy standalone that happens to feature a central romance between two princesses is in and of itself somewhat groundbreaking. So huzzah for a book that handily demonstrates that this totally works! May it pave the way for more to come. 
 We get alternating POV chapters from Denna, a responsibly-minded princess arriving in the kingdom of her betrothed prince for a politically significant marriage while trying to hide her magical fire powers, and Mare, said prince's rebellious older sister, who likes to train horses and sneak into town dressed as a boy. Soon a mysterious assassination occurs, the beginning of a chain of events that only Denna and Mare may be able to get to the bottom of--if they can get past their differences. 
 The book suffers a bit from what I think is some debut novel enthusiastic-but-rough-around-the-edges-ness. But I'm generally happy to forgive such flaws if a story engages me and makes me care about the characters, and this one really did! 
 The worldbuilding, for example, is extremely generic. It's fantasy kingdoms with palaces and gowns and taverns and names with more letters in them than absolutely necessary. And most of the story relies heavily on all of the officials who run the government of this kingdom being so spectacularly incompetent that they can't even arrange an investigation of an assassination, and are willing to leap to conclusions with zero evidence (not that this doesn't happen in real life, but it happens in more complex ways, and it doesn't feel like Coulthurst is going for that type of political commentary here). This is necessary, of course, because our under-appreciated teenage heroines need to be the only people smart enough to save the day! But Denna and Mare are both genuinely likeable characters, making it easy to root for them, both individually and as a couple, and the story is fast-paced enough that I never had time to linger on its faults. 
 I feel like I should mention that there doesn't seem to be any real homophobia in this world, or at least not in the parts of it that we see in this book. The obstacle to Denna and Mare's romance is not that they're both girls, but that Denna has had this incredibly important marriage alliance lined up for her since childhood. It's supposed to make her a queen some day, and she's very dedicated to that path in life. I personally don't identify anywhere on the LGBTQ+ spectrum, but it seems to me like Coulthurst has chosen to use Denna's struggle to hide her magical affinity to speak to teen readers who may struggle with being closeted, while at the same time portraying same-sex relationships as something completely normal and accepted. 
 I found myself comparing this book to Kristin Cashore's Graceling (and sequels), another series that intentionally inserts healthy, progressive romantic relationship dynamics into otherwise traditional fantasy settings and stories. Both Of Fire and Stars and Graceling suffered from some of the same debut weaknesses for me (generic fantasy world and magic system, contrived plot, etc.), but I forgave them both for many of the same reasons. And Coulthurst employs an intrigue-mystery-at-court type of storyline that reminded me strongly of how Cashore's books are structured. So I'd recommend this book if you're a fan of the Graceling Realm series!