Of course, the novel I must start out with is Ann Radcliffe's The Mysteries of Udolpho (1794). It's mentioned 18 times during the course of NA, and the author five times, so it's the one that engendered the most curiosity. It's the book that Catherine and Isabella exclaim over: there is a mysterious black veil with something shocking behind it, and Isabella proclaims that once Catherine is finished she will give her a dozen more books that are just as "horrid." So I was expecting something dreadful (by tame 18th century standards), a novel of suspense and horror.
Sigh. What I got was endless pages of a) descriptions of the sublime (lots of scenery and peasants dancing); b) regular bouts of poetry (mostly inspired by the sublime); c) overly passionate characters on the verge of collapse (physical and emotional); and d) endless hint after hint of secrets to be revealed, which of course weren't revealed until the very last chapter. It's not that I have any objection to being teased by a book, but if you're going to spend 54 chapters on a novel called The Mysteries of Udolpho, you should at least mention Udolpho before Chapter 3 of Part 2. (And let me add that these weren't short chapters by any stretch of the imagination; although I read an e-book from the awesome Project Gutenberg, hard copies run at least 700 pages.)
Instead, the first volume is devoted to showing us the St. Aubert family of France as the father and daughter, Emily, lose their beloved mother and take a trip along the Pyrenees for a change of scenery. And oh, boy, do we gets lots and lots of scenery, all of which uplifts the St. Auberts' souls exactly as the British philosophers of the time said the sublime should. During the trip they meet a very nice young man named Valancourt and travel with him for a few chapters, until Msr. St. Aubert's health fails (another few chapters) and he expires in a little town near an abandoned chateau where mysterious music is often heard. His dying instructions are for Emily to burn some of his papers without reading them, and of course she sees a glimpse that horrifies her, but she burns them before finding anything else out (or revealing it to us). Emily mopes to the point of fainting for two more chapters, until she is put in custody of her aunt, whereupon she mopes for two more chapters in Paris. Her aunt at first refuses to let her see Valancourt, then promotes the match when she discovers it would be socially advantageous. Emily allows herself to fall in love, but her aunt's sudden marriage to the sinister Montoni cuts the relationship short, as he takes them all to Italy.
We get many many more sublime scenes (many of which inspire poetic verses) as Emily crosses the Alps, goes to Venice, and is finally—after some 17 chapters—taken to Montoni's stronghold, Udolpho. There he imprisons Emily's aunt to force her to sign over some property, threatens Emily after her aunt dies, heads his own army (which borders on being brigands), and generally makes life rotten. The mysteries of Udolpho are mainly: what happened to the woman from whom Montoni got the castle; what is behind the black veil that Emily glimpses (but doesn't describe fully until the end); and who is making the mysterious music she can hear at midnight?
It turns out the music is from an imprisoned Frenchman who is not Valancourt, but he escapes with Emily, her maid, and the maid's boyfriend. They end up being shipwrecked near the town where Emily's father died, and there (after 35 chapters!) we are introduced to a whole new family: the Count de Villefort and his daughter, who now occupy the mysterious chateau. They befriend Emily, who sadly learns that Valancourt has fallen into vice and gambling and debt and is too sinful for her to consider marrying any more. They take two chapters to say goodbye, and Emily mopes for the rest of the book, even as the Frenchman who rescued her waits in the wings. We get more mysterious music, appearances, and hidden identities at the Chateau, until all is resolved in a single chapter, after a dying nun reveals her relation to Udolpho and Emily's relation to the Chateau. Valancourt turns out to be falsely accused, and the two can get married, which is good, because they've been so miserable without each other it's been miserable to read about them.
As for the mysteries? Nothing is supernatural or dreadful; what's behind the black veil is a trick (if I had to read 700 pages to discover exactly what, you do too), and hidden passages account for others. It's not shocking or scary at all, which I suppose I should have known from reading NA. After all, Henry Tilney—a man of the cloth—admits to reading and enjoying the novel, while John Thorpe considers it uninteresting. Unfortunately, I have to agree with the scoundrel ... if you're going to hold my attention through 700 repetitive pages of the sublime, you need to show me what's scary, not just show me someone else being scared ... especially if that character is an overly emotional, silly girl with no backbone of her own.