Ian Watt, in particular, is useful in understanding more broadly the significance of naming in Clarissa. The Rise of the Novel's introduction is interesting in how it suggests that the novel presents a sharp break with literary history when it comes to names:
Characters in previous forms of literature were usually given proper names; but the kind of names actually used showed that the author was not trying to establish his characters as completely individualised entities...The early novelists, however, made an extremely significant break with tradition, and named their characters in such a ways as to suggest that they were to be regarded as particular individuals in the contemporary social environment. (25)
The second argument that I find compelling is the use of names as they relate to the titles of novels, and the argument on sexual inequality. From "The Naming of Characters in Defoe and Others":
The use of a single, Christian name for women, but of a full name for men is part of a tacit discrimination between the sexes which is traditional in the novel, and which is reflected in the traditional novel-title. Against Joseph Andrews, Tom Jones, Sir Charles Grandison, Tristram Shandy, Roderick Random, we can set Pamela, Clarissa, Amelia, Evelina, Emma. (333)
What I'm also interested in thinking about--in addition to the way that names say something about the novel itself--is also how the name points to a certain confusion of identity, and how that is linked to the epistolary form of the novel. Many critical scholars--particularly Terry Castle and Stephanie Fysh--suggest that the materiality and form of the novel intervenes in the "illusion of realism" that Richardson achieves. In other words, there versimilitude of reality seems to work on a closed-circuit, and the formal quirks of the novel (Paper X, the signatures that are not signatures), this reality is laid bare as illusion for the reader. This may be what Lacan would refer to as a méconaissance--a misrecognition of the self and of reality itself. Following this psychoanalytic strain, I would suggest as a secondary source, Slavoj Zizek's Enjoy Your Symptom! Jacques Lacan in Hollywood and Out. In the first chapter he suggests why the recipient of a letter always recognizes himself as the addressee of a letter; in other words, the recipient uses to letter to "misrecognize himself", and the letter is essential in constituting individual identity. Unsurprisingly, the first chapter is titled: "Why does a letter always reach its destination?" and in it, he writes of the letter recipient:
I don't recognize myself in it because I'm its addressee, I become its addressee the moment I recognize myself in it. This is the reason why a letter always reaches its addressee: because one becomes its addressee when one is reached. The Derridean reproach that a letter can also miss its addressee is beside the point: it makes sense only insofar as I presuppose that I can be its addressee before the letter reaches me--in other words, it presupposes the traditional teleological trajectory with a preordained goal. (12)
Whew. Well, hopefully I didn't get too far off course with this posting, and hopefully raised a few more ideas to consider when it comes to letters, names, addressees, reality, and identification.