I'm halfway through this book & I absolutely love it. (Unless the end ruins it for me. Ever have a book do that?)
It's actually a spinoff of Rainbow Rowell's earlier book "Fangirl." That book's main character Cath is a huge fan of of a "Harry Potter"-like series about a boy named Simon Snow and his adventures at mage school with his best friend Penelope, his girlfriend Agatha, and his nemesis Baz.
"Fangirl" is a great book too, by the way. So, it's about Cath going to college and trying to finish her Simon Snow fanfic before the seventh book comes out. Throughout the book, there are excerpts of this Fic, which is called "Carry On, Simon Snow."
So, "Carry On" is the Simon Snow story. It's the final book of a nonexistent series that really isn't Harry Potter. And it's amazing.