Enna Burning, by Shannon Hale
In the first book, it was explained that everything has a language—Isi is able to speak to birds as well as to the wind. (And telepathically to her horse, but there's a different reason for that.) In Enna Burning, Enna the Forest girl (Isi's confidante and best friend) learns the language of fire.
It was a wonderful sequel. The love story between Enna and Finn alone made it worthwhile—although I've been pretty sappy lately, so that might just be me. (Finn has been added to my List—which I'm going to have to write down at some point.) Fire is a dangerous and powerful element to play with—even more dangerous than wind, which may be slowly driving Isi insane.
Bayern is attacked by a neighboring country, leading to war. The author certainly didn't pull any punches—fire used in battle leads to ugly, ugly descriptions. At first I didn't think I was going to enjoy it quite as much, but by the time Enna was captured by enemy forces, I was completely won over.
I especially enjoyed the relationship between Isi and Enna—there was a really nice forgiveness piece that was realistic, emotionally true and NOT a bash in the head (unlike Mira, Mirror which, in hindsight, is only beginning to irritate me more and more). The magic system is really well done and original—although it isn't quite magic; it's more a different kind of communication—and Shannon Hale did a great job exploring the different ways that the elements could work together. I'm wondering if there'll be two more books—she could easily round out the series with Earth and Water. (As long as she skips 'Heart', thus avoiding all comparisons to that beastly Captain Planet show.)
Even more than what she did with the elements, I loved the further exploration of people-speech, which, in the right (or wrong) hands, seems more dangerous than anything that Isi or Enna can do. There's a character who is capable of more impressive manipulation than I think I've ever seen. I find that creepier than any sort of magic.