Books, especially Gothic novels, are important symbols here. They are so important that they symbolize, or represent, a lot of different things. So there are a ton of different ways to interpret books and Gothic novels in Northanger Abbey.
Books and Gothic novels play an important role in a lot of the books' major relationships. Catherine and Isabella bond over Gothic novels. Books also help us to better understand the characters. For instance, both John and Henry read Gothic novels, though John insists that they are stupid, while Henry insists that people like John are stupid for thinking such things.
The way characters talk about novels and read them often provides insight into their personalities. We can see a definite contrast between John and Henry based on their reading habits. Here are John's views:
I took up the first volume once and looked it over, but I soon found it would not do; indeed I guessed what sort of stuff it must be before I saw it [...] I was sure I should never be able to get through it. (7.40)
John judges too quickly here and goes on to bash a book that he hasn't actually read. He basically sounds ill-informed and ridiculous. John also calls Gothic "stupid," even though he has read a lot of them. Henry, meanwhile, has very different reading habits:
I have read all Mrs. Radcliffe's works, and most of them with great pleasure. 'The Mysteries of Udolpho,' when I had once begun it, I could not lay it down again; - I remember finishing it in two days - my hair standing on end the entire time. (14.7)
Henry sounds a lot like Catherine here – he enjoys reading and finds Gothic novels fun and exciting. So the way characters discuss books and read them reveals a lot about who these characters are. Books, and Gothic novels especially, also play key symbolic roles in the books' various relationships. Catherine, after all, discusses Gothic novels with nearly everyone in the book.