I devoured this book. I adored the protagonist, Cath, with whom I may or may not have over-identified. This led to some intense passages for me, or essentially an intense read overall.
The book follows Cath and her twin sister, Wren, through their first year of college in Nebraska. The two are huge Simon Snow fans (read: Harry Potter with a dash of Twilight, though HP is mentioned in text) and used to write fanfiction together. Cath still writes it. She is the introvert to Wren's extrovert. Wren is determined to be her own person, and the two live in separate dorms on campus and start leading separate lives. We know this is hard on Cath.
There are difficulties with their most likely bipolar father, their MIA-and-now-reappeared mother, guys, roommates, drinking, schoolwork. It's angsty, it's funny, it's sweet, and in the end, it's happy. Loved it.
The author does a good job not pathologizing Cath (and Wren and others) for being fans and writing fanfic. Cath isn't an introvert because of that. And one of my favorite bits is when Cath (who is a Creative Writing major) mentally criticizes her "writing partner," Nick, for writing a "Mary Sue," one of those girls that don't exist in the real world that men like to write--perfect and quirky (I'm thinking of that movie Ruby Sparks). Right on.