Northanger Abbey is largely a novel about novels and about reading habits. This book has an odd sort of self awareness of itself as a novel and the narrator often steps back from the story she's telling to consider it as a story. Rather than just tell us about Catherine, we learn about Catherine as a heroine and as a fictional character. Gothic novels are lampooned, or made fun of, here as well, and the text considers the oftentimes disastrous effects of certain kinds of reading habits. It's a rather complicated venture, but Northanger Abbey not only considers the effects that literature has on its characters; it also considers the effect Northanger Abbey the novel has on its real world readers.
Though Catherine traces her mistakes back to the Gothic novels she read, her reading habits are really another symptom and not the cause of her behavioral issues.
Northanger Abbey is not about the problems of reading too much, but is instead about the problems of reading books in a certain way – i.e., too seriously.