This is from the National Book Festival yesterday. I also saw her presentation (which I can describe if anyone is interested). She largely answered questions, but veered off into "I wanted adventure and action and puberty, and I wanted to read about girls".
Which was a great place to veer off into if you ask me.
Battle Magic is a lovely book detailing Briar's time before Will of the Empress. However, you get into the heads of Rosethorn, Evvy, and several other characters. I won't say more since it comes out tomorrow.
Long story short, it's a wonderful addition to the Circle of Magic series and is released tomorrow (24 September 2013).