There are a number of passages (some of which you can read on Google Books at the link above) which are worth excerpting from the introduction and the first chapter on these subjects, but for now I'd like to flag a very interesting passage I found somewhat later:
In a provocative analysis of the 'middlebrow' novel of the 1930s, the 'realist' novel with an ambiguous relation to the 'literary,' the Birmingham Centre's English Studies Group has argued that the reader of the middlebrow novel is characteristically 'interpellated in the position of the author or narrative "point of view".' The 'lowbrow' or mass formulaic novel "interpellates' or situates its reader in a position of identification with one or more characters, and the 'highbrow' or modernist novel situates the reader in the position of literature itself. These various situations are tied, the English Studies Group argue, to the different educations of different classes and reading publics in the school system. (64)Interpellation is a fairly sophisticated concept, but here I think we can paraphrase it to mean something like "situated"—the reader of the middlebrow novel is situated to view the action from the narrator or author's perspective, the lowbrow reader from a character's perspective (but not a character in control of the narration of the story). The highbrow reader sits in the place of literature—that is, in an abstracted position that always treats the question of point-of-view as in itself problematic, unstable.
A lot can be said about this division, particularly given that it is set up to describe 1930s fiction and so naturally needs an updating. Beyond the obvious problem that your middlebrow novel might be my borderline-highbrow novel (or vice versa), I think the really interesting issue comes in with how certain novels of recent years have very explicitly tried to cross these distinctions, such that the reader is supposed to believe for a time that they are reading a middlebrow novel, and then something gets knocked out of the story, and the narrator-identified point of view that they've been occupying becomes suddenly untenable.
Perhaps the most famous occurrence of this is in Atonement, but one can also feel something like it in, say, much of Aleksandar Hemon's fiction and Kazuo Ishiguro's novels. This phenomenon is more, I think, than just an unreliable narrator, but has to do with the traditional associations of different types of point-of-view with specific audiences, in the way described above. So we get what seems like a diecast New Yorker short story from Hemon, and by the end find that reading it that way simply isn't sufficient (the irony being that it's still in The New Yorker in most cases). Or, in Remains of the Day or Never Let Me Go, we realize that the "problem" of the unreliable narrator is actually going to necessitate more than just a revision of the "facts" of the narrative, but a fairly disorienting detachment from the idea of being able to "view" the action in the first place.
At times, I think David Foster Wallace actually takes his reader in the opposite direction: convincing them that they're reading a 'highbrow' modernist novel par excellence, where the question of point-of-view is always problematic and the reader mustn't fall into the trap of identifying with one point-of-view. And then he basically makes you commit to a point-of-view: I question whether anyone can get through it (and enjoy it) without doing so. And that doesn't mean that you pick a character to empathize with for the rest of the novel, but that you have to create a position of provisional coherence from which to view the events and data of the novel and process them—whether that is identified with a character or with the author or with some external position. So by the end, you're just reading a very complex "middlebrow" novel, if we're using the terms from the blockquote above.