"Perhaps you are not sitting in this room, and I am not sitting by you. These are points in which a doubt is equally possible. Not keep a journal! [...] How are the civilities and compliments of every day life to be related as they ought to be, unless noted down every evening in a journal?" (3.26-7)
Henry jokingly links journals to civilized life. The journals Henry speaks of are extensions of polite society, with certain rules that are followed.
Yes, novels; - for I will not adopt that ungenerous and impolitic custom, so common with novel writers, of degrading by their contemptuous censure the very performances, to the number of which they are themselves adding - joining with their greatest enemies in bestowing the harshest epithets on such works, and scarcely ever permitting them to be read by their own heroine. (5.4)
Lots of novels don't let their heroines read novels, but Northanger Abbey prominently features Catherine reading novels. Catherine is "corrupted" by her reading habits, much as these other heroines feared they would be. Catherine's corruption is not a serious matter though. Instead it is funny.
"Oh! I am delighted with the book! I should like to spend my whole life in reading it. I assure you, if it had not been to meet you, I would not have come away from it for all the world." (6.7)
Catherine is showing signs of being obsessed with Gothic novels here, since she is reluctant to go out and socialize.
"Udolpho! Oh, Lord! not I; I never read novels, I have something else to do."
Catherine, humbled and ashamed, was going to apologize for her question, but he prevented her by saying, "Novels are so full of nonsense and stuff; there has not been a tolerably decent one come out since Tom Jones, except the Monk; I read that t'other day; but as for all the others, they are the stupidest things in creation." (7.33-34)
John Thorpe isn't exactly an authority on anything, and his sentiments on novels are undermined by the fact that he does in fact read them. Notably, Catherine is embarrassed to like novels in the face of John's opposition. She's something of a closet fan.
Catherine was then left to the luxury of a raised, restless, and frightened imagination over the pages of Udolpho, lost from all worldly concerns of dressing and dinner, incapable of soothing Mrs. Allen's fears on the delay of an expected dress-maker. (7.54)
Gothic novels have a major, and humorous, effect on Catherine. She's basically oblivious to the entire world while she's reading. But she also takes getting lost in a good book to a potentially dangerous level, given what Gothic novels do to her imagination. The diction, or word choice, here indicates that Catherine's imagination is getting really worked-up.
"What, is it really a castle, an old castle?"
"The oldest in the kingdom."
"But is it like what one reads of?"
"Exactly - the very same."
"But now, really, are there towers and long galleries?"
"By dozens." (9.24-31)
Catherine is overly anxious to visit a castle with the Thorpes in order to experience something she has read about. Ironically, the Thorpe's are actually lying about this castle being old, so Catherine would not have had much of a "Gothic" experience if she had gone.
"But you never read novels, I dare say?"
"Because they are not clever enough for you - gentleman read better books."
"The person, be it a gentleman or a lady, who has not pleasure in a good novel, must be intolerably stupid." (14.4-7)
Henry makes a statement about the book's sentiments about novels here. Interestingly, Henry notes that novels cross gender lines and are enjoyed by both women and men. Only obnoxious or stupid people would dislike them.
"I wish I were too. I read [history] a little as a duty, but it tells me nothing that does not either vex or weary me. The quarrels of popes and kings, with wars or pestilences, in every page; the men all so good for nothing, and hardly any women at all - it is very tiresome; and yet I often think it odd that it should be so dull, for a great deal of it must be invention [...] and invention is what delights me in other books." (14.22)
This passage is unusual in that it is Catherine, and not Henry, expressing these sentiments. Henry usually serves as the narrator's mouthpiece, but in this case Catherine does with her hilarious observations on male-dominated history. Catherine also reemphasizes her love of invention and imagination here as well.
Her passion for ancient edifices was next in degree to her passion for Henry Tilney - and castles and abbeys made usually the charm of those reveries which his image did not fill [....] Northanger turned up an abbey, and she was to be its inhabitant. Its long, damp passages, its narrow cells and ruined chapel, were to be in daily reach. (17.11)
Catherine's obsession with "Gothic" abbeys and castles has reached a fever pitch here. Her expectations for Northanger Abbey are largely colored by what she has read
He smiled and said, "You have formed a very favourable idea of the abbey?"
"To be sure I have. Is not it a fine old place, just like what one reads about?"
"And are you prepared to encounter all the horrors that a building such as 'what one reads about' may produce? - Have you a stout heart? - Nerves fit for sliding panels and tapestry?" (20.11-13)
Henry makes fun of Catherine's tendency to confuse fiction with reality here, noting that "what one reads about" is not likely to happen in reality.