I got my hands on an advanced copy a few days before it came out, and from 8:30pm to 5am I read Carry On and did very little else. I think I grinned the entire time; it was definitely worth getting so little sleep over.
In its bare bones, Carry On is Harry Potter fanfiction that lovingly pokes fun at the source material. It takes place in Simon Snow's final year at magic school and references past years as if previous books in the series existed, (including scenes explained in more detail in Fangirl) but it works very well on its own. It is just as wonderfully fluffy as the rest of Rainbow Rowell's books, and is well-balanced between the external action and character development.
Carry On is also really refreshing in its representation of different minorities. There are main characters who are bisexual or gay, minor offscreen lesbians, and a variety of different races and classes.
Readers of this may also enjoy: The Darkest Part of the Forest by Holly Black, The Iron Trial by Holly Black and Cassandra Clare, The Magicians Trilogy by Lev Grossman, and A Hero at the End of the World by Erin Claiborne.— Nicole