I decided to read this after hearing good things, enjoying Attachments, and the fact that there is a fanfiction subplot. I've read fanfiction off and on for most of my life. I remember when fanfiction.net did not have chapters. I will admit to co-writing one as a kid with a friend and I hope it no longer exists since it was horrible.
Fangirl follows Cath as she starts college with her twin sister Wren. Cath writes fanfiction for a popular children's series Simon Snow (basically Harry Potter, although Harry Potter does still exist in this world) and her fanfics revolve around Simon and his school mate Baz. Basically Harry and Draco.
There was a lot in the book I could relate to. Cath is an introverted person who frequently survives on protein bars so that she does not have to go to the dining hall to eat. She was hard to root for at times because she seemed so determined to shoot herself in the foot which was frustrating. Overall, I enjoyed the book and plan to read more of Rainbow Rowell's books.