Wildfire is a lovely book, surpassing even its predecessor in my estimation to earn a full 5 stars. The only downside to this book is that it isn't standalone: Firethorn should be read first, and we are still awaiting the third installment.
My favorite aspects of this trilogy are its focus on the lives of women, particularly in the lower classes, and its worldbuilding. Both are prominent here. The heroine Firethorn travels to new lands with fully realized cultures, including one that's reminiscent of ancient Egypt. I don't know if the author has an anthropology background or not, but I'd be willing to believe that she does; rarely have I seen worlds as unique yet believable as this. However, this is still a gritty fantasy and doesn't shy away from the harsher aspects of the cultures it examines. It raises questions about how women and the lower classes have been treated throughout history and provides plenty of food for thought.
Furthermore, I would say the characterization improves with this book: Firethorn becomes a more morally ambiguous character, and a variety of other interesting characters are introduced. I agree with other reviewers that the plot meanders a bit: more happens in this installment than the last (as it should, since it's noticeably longer), but Micklem seems at least as interested in thematics and taking us through her world as she is in the destination. I wouldn't recommend this book to those who require lots of action scenes. Finally, the quality of the writing is very good; I wouldn't expect even literary readers to have complaints on that score. As far as fantasy elements go, as in the first book the supernatural remains mostly in the background.
In sum, Micklem is one of the best new fantasy authors I've found, and I hope she doesn't keep us waiting too long for the conclusion to this trilogy. Highly recommended.