This book is technically a prequel to Cashmore's first novel, Graceling, but it's set in a different land, with very different magic, and featuring only one character from the other book, so it is easily readable even if you haven't read Graceling (although if you're already planning to read both, don't read this one first).
I was surprised to see that the prologue to Fire is one of the creepiest I've ever read... but the creepiness didn't continue into the book proper, so don't let that put you off. The character Fire is both similar to, and different from, the main character in Graceling: like Katsa, Fire is initially strong and scared of her own power, and gradually grows to accept herself; but she's a bit less independent than Katsa, and she desperately wants children (but refuses to actually have any because she would pass on her powers to them). She doesn't have an easy life: both humans and animals are affected by her, and either hate her and try to harm her, or want her so much that they smother her. She's also affected by the memory of her father, who was a brutal man who abused his powers and caused chaos in the country - although he loved her and never mistreated her.
There's an inevitable romance that I saw coming from very early on; but it's handled excellently and the story is not focused upon the two of them. The plot is interesting and exciting, and the writing is every bit as competent as in Graceling. I found the novel so gripping that I don't think I put it down for more than a few minutes at a time until it was finished.