[This snippet is actually from the letter exchange, Terry Castle responds to a reader, condensing the viewpoint to a nutshell]:

[. . .] I would not describe The Blazing World as a first-rate novel. But I have to say I concur with the rest of his opening salvo: I did find myself hankering for things the novel failed to provide. Among them: true wit and intellectual depth (as opposed to the author’s relentless grad school preening), a minimally plausible story line and believable characters, some appreciable emotional resonance, and—how exactly to put it?—whatever
it is that makes a good novel all of a piece: delightful and risky and alive and worth caring about.

It’s no defense of Hustvedt to say that the problems I mention arise because I’m
too rigid to see that she is “ambivalent” about her tedious heroine and “knows better than to invest uncritically” in Harriet/Harry’s cartoonish anti-male views. Authorial ambivalence—about anything—has nothing to do with a book’s readability. And unlike Nabokov or Woolf, two of the great masters of literary multivocality, Hustvedt often just seems confused—unable (technically or emotionally) to manage all The Blazing World’s moving parts. By the end the whole lumbering, lurching juggernaut goes quite spectacularly off the rails.

Faced with what I suspect is a basic weakness in the conception of the work,
the novelist’s recourse is to confound matters further by heaping on gratuitous literary and philosophical references—the more recherché the better. Margaret Cavendish, Harriet’s revered “Blazing World Mother,” is one of a cast of thousands. Witness this passage in which Harriet’s daughter describes her peculiar parent:

I don’t think anybody really knows when she first started thinking about pseudonyms. She published one dense art review under the name Roger Raison in a magazine in the eighties, dumping on the Baudrillard craze, demolishing his simulacra argument, but few people paid attention. I remember when I was fifteen, our family was in Lisbon, and she went over and kissed the statue of Pessoa. My mother told me to read him, and, of course, he was famous for his heteronyms. She was also deeply influenced by Kierkegaard.

It’s not because I don’t “get” the references ostentatiously piling up here—Baudrillard on Disneyland, Kierkegaard’s aliases, Pessoa and his heteronyms, or, indeed, the whole Mommy’s-kissing-a-statue saudade of it all—that I find Hustvedt’s name-dropping way of characterizing her heroine coy and insufferable. A Little Wikipedia Is a Dangerous Thing. The fact is, the book is pretentious and contrived to the point of readerly burnout. It is also (dare one say) often dead-in-the-water boring.

Siri Hustvedt is far too subtle and multifaceted a raconteur to present us with a simple tale of institutional misogyny. “The path to the truth,” in her heroine’s words, “is doubled, masked, ironic.” Harry Burden, as revealed through both her own testimony and that of others, is a self-sabotaging bundle of confusion. “Loud, lecturing, unpleasant,” prone to haranguing or snapping at people who might be useful to her, alternating between gawky silence and explosive rage, the 6-foot-2-inch-tall Harry looks “like a cartoon character, big bust and hips, . . . a galumphing jump-shot-sized broad with long, muscular arms and giant hands, an unhappy combination of Mae West and Lennie in ‘Of Mice and Men.’ ” In one of the novel’s most heartbreaking passages, Burden’s childhood friend, now a psychoanalyst, tells us that the literary character with whom the teenage Harry most identified was Frankenstein’s monster. “The terrible being Frankenstein makes is so lonely and misunderstood that his very existence is cursed. . . . His awful isolation is transformed into vengeance.”

Hustvedt has constructed the novel as a kind of artefact, out of numerous kinds of testimony: it purports to be the work of an academic researching Harriet Burden's claims of authorship years after her death, and is a collection of interviews, essays, articles and letters demonstrating the spectrum of responses to the would-be scandal. . . .
. . .
Hustvedt has a lot of very entertaining satirical fun in The Blazing World, but that particular note of tragedy, though she tries to sound it, remains lost.