Likely, I'm just too dense to get it. But this seems a case of incomprehensibility substituting for profundity.
The story os about a hack science fiction writer, Jonathan Herovitz, who turns out a dreadful series about Mack Miller under the pseudonym Kirk Poland. All the names are important, 'cause, ultimately, the book is about an identity crisis.
Herovit is having a hella hard time adapting to the new demands of science fiction publishing at the end of the 1960s, which is asking for unusual plots and literary competence. He still has a market, though, and has recently signed a contract with a large publishing house that seems intent on wrangling the last nickels out of the series. He's having writer's block, though, and simply cannot go through the motions any more. He's also having trouble with his wife--he has a series of casual affairs--and is drinking too much. His agent if riding him. His frenemy--an ex-science fiction writer who has started teaching at a college--is goading him.
And so as his world collapses, he finds his personality disintegrating, his self replaced first by the pseudonymous author--but that proves a shell, too easily pierced by those he is trying to fool. His agent might be confused, since he can only talk to him on the phone, but those who actually see him still know it's Jonathan Herovit. Which is when he starts hearing Mack Miller in his head.
Mack Miller, his science fictional creation, is the leader of a rough-and-ready survey team that visits unknown worlds, mostly to destroy them. If it's possible to spoil a book that is four decades old, here's a spoiler: Jonathan then starts thinking he's Mack, and the world around him--earth, our earth--is alien. He must destroy it: of course, without Mack Miller's special skills and technology, he can do know such thing, and so the book ends with Herovit being destroyed.
There is a lot of meta-commentary on science fiction itself. Herovit is kicked out of a science fiction league by a thinly disguised A. E. Van Vogt--who's intrigued by a thinly-disguised Scientology. Mack Miller's team is the antipodean version of Star Trek--killing rather than bringing peace, but still an on-going series, the very definition of genre entertainment--which makes the pseudonymous author's name Kirk resonant. Probably there are more inside baseball references I just missed.
The biggest problem with the book, though, is the language. For one thing, it is very dated--it says late '60s early '70s all over it. Another is that it is wooden and pulpish. The sex scene is a rape scene and it is unclear to me whether the actual author--Baryy N. Malzberg--knows this. Herovit's descent seems clunky--forced--and there are way too many dream sequences. This is where lack of clarity--simply making unexplained jumps--seems to substitute for profundity.
One might make the case that the book is purposefully pulpish, since it's being told through the point of view of a pulp writer: the way, for example, Kennifer Egan's "The Keep" is purposefully poorly written at the beginning. But Egan has a pay-off for the reader willing to take up her bet. The reward is worth the risk. There's really no pay-off here--other than Herovit becoming a pulp creature he has always ben--which means that one has to put up with a lot of clunky, wooden writing. Not sure it's worth it in the end.