My first advice to any potential reader of this book: don't read the synopsis. It tells you far too much about what happens in the story.
Those who have read "The Goose Girl" will have already met Enna, Isi's fearless and outspoken friend. It's not necessary to have read TGG first, but it does help to understand references to past events in this book. It also helps the reader to understand Isi, Enna's devotion to her, and the background to the situation at the beginning of the book, as well as the various powers mentioned.
"Enna Burning" picks up a few months after the end of the first book. The book is divided into four parts, charting Enna's spiritual journey. At first, there is a mysterious prologue, which becomes rapidly clear during the first part of the book, as Enna's brother Leifer begins to act oddly, eventually trying to burn her alive. This domestic unrest is mirrored in the wider situation, as Bayern is threatened by a neighbouring country, Tiran. Enna heads back to the city to be with Isi, now Queen, but she finds Isi is not as she once was: her wind power, once a blessing, is rapidly becoming a curse. Enna's wish to help Isi and protect Bayern leads to her making some reckless decisions, and she is eventually captured. This is probably the most interesting part of the book, as Enna is forced to confront her actions and whether she can survive without her fire gift. I won't say any more, as I don't want to spoil the ending, but I felt very happy and fulfilled after finishing this book. It has a slow beginning, but quickly gathers pace and ends on a wonderful note. Best of all, it made me want to go back and read "The Goose Girl" again.
Highly recommended.