Others who are smarter and better read will be better able to answer the following question:
Why hasn't Sofia Western read Pamela? or Clarissa?
And this question, too:
Why hasn't Clarissa read Pamela?
Maybe even this question:
Why hasn't Arabella read either?
Or this question:
Why hasn't Miss Betsy Thoughtless read any of 'em?
You might say that just because they don't mention them doesn't mean they haven't read them. I might respond that given their effect on the genre and their involvement in a very presentist discourse about the education and entertainment of young women, they're conspicuously absent. The novel as it came to prominence during the 1740s and 50s seems to be missing from the novels of the 1740s and 50s.
I am the first to admit that I haven't read nearly enough to make this assessment with any certainty. I can say that in what I have read, there seems to be no diegetic cross-referencing--no acknowledgment by characters or narrators of the presence of those novels that were trying to do away with the pernicious novels of Behn, Manley, and Haywood (in her 1720s incarnation). It occurs to me that Charlotte Lennox, whose The Female Quixote is specifically and explicitly about what should and should not be read, might have mentioned a contemporary title or two by name. Samuel Johnson thought highly of Lennox; he likewise thought highly of Richardson. Why not plug the latter or offer Clarissa up as an acceptable substitute for all those ridiculous romances buckling the shelves and warping the minds of anyone who comes into contact with them?
The novel, for all its formal realism and pretensions to representing a world sans fairies and dragons, seems to have left itself out of the world it purports to depict. Does anyone out there know offhand when Tom Jones or Clarissa shows up in the library or hands of another character in a novel?
I suppose that each of the novels I have mentioned here set out to redefine the genre, and would therefore not be served by giving free press to a competitor. Perhaps it's simply a function of sustainable fictiveness--it always makes me slightly uncomfortable when characters on TV talk about shows that they watch (I just saw a rerun of the Seinfeld episode in which Jerry is accused of watching Melrose Place and is caught out by a polygraph test). There's something very disconcerting about hearing that theme music within the text of another program. (That's right, I said text. Cut me some slack.)
Maybe it's about elevating the novel in the hierarchy of genres. Novelists are happy to talk about old novels, and they quote freely from plays, poems, sermons, essays, and so on. Interestingly enough, though, even when they do make a reference, it's rarely to a contemporary piece. Pope gets quoted a lot, of course, but by 1749 (Tom Jones) he'd been dead for a few years and in any case most of what gets quoted is from poetry he wrote before 1730. Milton makes his contributions. Dryden was quite popular with Haywood; so was Edmund Waller. All men, and all well and truly dead by 1750. It's possible that the unattributed snippets are contemporary, and there's at least one occasion in MBT when Haywood interpolates a ballad fresh from the street, but the fact that they're given no attribution by the narrator or editor suggests that wherever they came from (unless composed by the author), the weren't thought enough of to find their own ways to fame and fortune.
In any case, I wonder if there's something there about the novel being self-contained, whereas all else is contained by the novel. That'd put it on top as a function of its ability to yoke the rest to its purpose.
Still doesn't really explain why Sofia hasn't read Clarissa.