During the school year being Simon Snow is pretty excellent. There's the whole attending magic school thing and getting to eat whatever you want thing. There are some drawbacks, like being believed to be the subject of a prophecy and sharing a room with a very confusing (and handsome?) probably vampire.
Carry On is the end of Simon's quest, the reader hits the ground running. It's awesome. You get right to the meat of the story and it means this is a super tight fantasy novel. Simon Snow is going to have to face his mortal enemy and his time is almost out. How could you not want to read that?
Do you remember when you first read Harry Potter? How good it felt to be in that magical world for the very first time, learning the way the magic works and meeting all the characters? This book is going to give you some of the very same feels and that is pretty amazing.
You might recognize the name Simon Snow from Rainbow Rowell's previous book, Fangirl, where the main character writes fan fiction about this world. If you haven't read Fangirl yet, you do not need to read it first, this is a stand-alone novel and works all on it's own. That being said, Fangirl is great and you should totally read it too.
One of my favorite parts of this book is the epilogues (which never happens), it's like Rowell took all my hopes as a reader for her characters and made them come true. That's magic all on it's own.