First, in print: it's available in hardcover and paperback from your favorite indie or world-eating bookstore. Second, it's available as an e-book. Same rules apply. Third, it's available in audiobook.
The Crown of Embers comes out next week, on the 18th. I'll review that tomorrow. But to fill the gap, there's a prequel short story available as an e-book, called The Shadow Cats. HarperCollins's website describes it thus:
Once a century, one person is chosen for greatness. And it was not Alodia.
Alodia is the crown princess of the realm. The sister who knows how to rule, the one who is constantly reminded that she has not been marked for a grand destiny. But Alodia has plans, and she will be the greatest queen her people have ever known. So she travels—with her hopeless, naive, chosen sister—to a distant part of their land, to begin to secure her supporters. This region needs its princesses, for it is plagued with a curse; the crops don't grow, spring doesn't arrive, and a fierce jaguar stalks in the shadows, leaving behind only empty homes splashed with blood. If Alodia can save them, no one will be able to deny her strength and her sovereignty.
But what she discovers could change the fate of her kingdom, if not the entire world. And it will most certainly change her opinion of her younger sister.
(I'm hoarding this one; I can't decide if I'll read it when it's halfway to the last book in the series, or if I'll save it for the very end.)
Rae Carson's website: http://www.raecarson.com/
Rae Carson talking about The Girl of Fire and Thorns:
Tomorrow, I'll review The Crown of Embers.