Julie Kagawa's Iron Fey Series took me by total surprise and I just couldn't get enough of them. The characters were like family and once the story was over, were sorely missed.
For fun, I went back to my first review of The Iron King and reread my reaction, which I've posted below the book covers. It totally encompasses how I feel about the entire series.
If you're a fan of Young Adult books and Fantasy in particular, this series is for you.
My Goodreads review of Iron King: