The Iron Knight by Julie Kagawa

Ash, former prince of the Winter Court, gave up everything. His title, his home, even his vow of loyalty. All for a girl… and all for nothing.
Unless he can earn a soul.
To cold, emotionless faery prince Ash, love was a weakness for mortals and fools. His own love had died a horrible death, killing any gentler feelings the Winter prince might have had. Or so he thought.
Then Meghan Chase—a half human, half fey slip of a girl— smashed through his barricades, binding him to her irrevocably with his oath to be her knight. And when all of Faery nearly fell to the Iron fey, she severed their bond to save his life. Meghan is now the Iron Queen, ruler of a realm where no Winter or Summer fey can survive.
With the (unwelcome) company of his archrival, Summer Court prankster Puck, and the infuriating cait sith Grimalkin, Ash begins a journey he is bound to see through to its end— a quest to find a way to honor his solemn vow to stand by Meghan’s side.
To survive in the Iron realm, Ash must have a soul and a mortal body. But the tests he must face to earn these things are impossible. At least, no one has ever passed to tell the tale.
And then Ash learns something that changes everything. A truth that turns reality upside down, challenges his darkest beliefs and shows him that, sometimes, it takes more than courage to make the ultimate sacrifice. From Goodreads.

I have really enjoyed this series of The Iron Fey by Julie Kagawa. This book, which seems to be the last of series, is about Ash and his quest to become human so he can join Meghan at the iron court.
As far as I can see, this is currently the last book in this series but it seems there is another series: The Iron Fey: Call of the Forgotten which is set in the same world starting with a book about Meghan's younger brother. I will continue with that series at some point.

This book qualifies for:
Goodreads 2013 Reading Challenge
Serious Readers 2013
Tower Teams Read 2013-2
2013 Ebook Challenge
Beat the Heat Readathon 2013
2013 Paranormal Reading Challenge (fey)
2013 Sequel Reading Challenge
Ten Book Author Train Challenge