The Iron King by Julie Kagawa

I love a good fairy book, and the addition of the new iron fey species was a good one. Fairies are created with human imagination, so, of course, humans now dream about technology. And the iron faeries are born. Meghan Chase is almost sixteen and has no idea that she is part-fey. She learns the hard way when her little brother is kidnapped and replaced with an evil changeling. Everyone and everything is not what it seems and she must face incredible challenges to try to save her brother.