One of the most interesting themes in the book is the idea of reading and how it can influence us--for better or for worse.
I recently read The Romance of the Forest by Ann Radcliffe, which made Northanger Abbey even more fun. Northanger Abbey's main character, Catherine, is obsessed with Ann Radcliffe's horror stories (rather like a Twi-hard fangirl today, actually). She's so taken by the Gothic trend of the time that she begins seeing evidence that one of her close acquaintances actually murdered his wife!...or worse...
I had fun trying to guess whether Jane Austen admired Radcliffe or detested her. Or somewhere in between--just rolled her eyes at her. They were, actually, close contemporaries; Radcliffe was publishing in the years just before Austen. Radcliffe was actually extremely popular at the time, but as the fangirl-ing in Northanger Abbey may suggest, wasn't always considered a serious author. She could probably be looked at as the Stephenie Meyer of the 18th- and 19th-centuries. (In public opinion only; I'm not making a statement about the quality of Twilight or Radcliffe's books, mind you.) Actually, at the time, novels were only just becoming very popular, and there were plenty of people who turned up their noses at novels, especially female "authoresses." (Can you imagine, somebody calling Jane Austen a sensation writer? Ha, ha! It's kind of funny to think about, considering Austen's reputation today.)
I think that Austen wasn't trying to make fun of Radcliffe; she was making fun of people who allow novels to interfere with their sense. (And sensibility! Oh wait, that's a different novel.) Austen is very sensible about reading. An avid reader herself, she doesn't actually pretend that all reading will make you a good person; if done in the wrong way or for the wrong reasons, it can actually hurt you. Catherine, bless her little heart, wasn't looking for ways she could improve herself through the characters' good and bad traits; she was allowing herself to get carried away and see things that weren't there.
One thing I thought was interesting about this book was that there was a focus on the "bad guys." Isabella and John Thorpe, two scheming, conniving, lying characters, got a lot more screen time than the Tilneys (the good guys) did. The lesson seemed to be in Catherine's reaction. I got the idea that the "moral" was: "hey, there are a lot of mean liars in this world and there's nothing you can do about it, so don't just blindly trust everyone you see." I wonder if Austen had some kind of experience when she was younger when she trusted someone and it backfired (maybe that cousin who was all interested in Austen's brother? Oh dear, I need to read up on Austen again). I can imagine it being a common problem for young women of the time period, who might not necessarily have been taught to have discretion in their acquaintances (unless it had to do with money or rank).
Another interesting thing about the novel was that none of the characters were particularly lovable. (Except for maybe Henry Tilney, but we just don't see enough of him to really like him.) Catherine Morland was just kind of eh. A lot of the time she was downright stupid. The funny thing is, though, Austen absolutely knew this and often pointed it out. (For example, at the very end, Mrs. Morland feels sorry for the house that has Catherine as a housekeeper, since she's not very good at focusing.) Maybe Austen was tired of all the popular novels with a practically-perfect-in-every-way heroine (cough, Radcliffe's books) and figured she might try something a little different--maybe something a little more true to life.
Northanger Abbey is great and quite different from Austen's other books. It's a very quick read and pretty light, without much plot but with a lot of humor. If you're an Austenite (or a closet Austenite) and you just haven't gotten around to this one, I suggest you do. It's not as romantic or as dramatic as Austen's other novels, but Austen's humor really comes through in it.