Untamed (House of Night Novels) by P. C. Cast and Kristin Cast
Book Four in the House of Night series - and another great addition! I think this one, though, is a bit darker than the first three in the series. As Zoey's power grows, it seems her battles get bigger. I love how she finds unusual allies in this book - but I don't want to say too much and give it away!
If you're reading this series, you'll love this one. And if you're not, you should!