Back in the late 1970s early 1980s the traditional SF magazines were in trouble and various experiments were tried to keep them going. One of these was to publish paperbacks with serialised novels i.e. the paperback would be like an anthology but instead of stories it would contain parts of around half a dozen novels. You would buy all the paperbacks to get the complete novels.
The novel I am trying to find was published in this form, but to make life harder I only had the second issue of the paperback so I missed the start of the novel and missed the conclusion. I read only a chunk from the middle of the novel, but it was intriguing enough that I would like to find the whole novel.
In the first part a man had had some form of alien encounter and as a result he was transforming into one of the aliens. At the start of the part I read he had been imprisoned in a secure facility so his transformation could be observed. At this point the man looked and felt normal, but the story described how when he felt the transformation starting he would crawl under the bed in his cell and the story described a golden light coming from under the bed as the transformation started.
The man would start transforming but then get frightened and the transformation would stop leaving him unchanged. Eventually he goes through it and transforms into a glowing golden saucer shape. The story described how his senses changed. For example in his saucer form he easily smashes through the armoured glass in the cell window and the story describes how he sees the shards of falling glass as some mathematical pattern. This is where the part of the novel I read ends.
There are some specific points I remember that might help with the identification:
the man has vivid dreams in which he is a female centaur like creature on some alien planet. The story describes a sexual encounter with a male centaur in gratuitous detail.
the man is estranged from his wife, and one of the psychiatrists at the secure facility describes him and his wife as: "like two rats trapped in a hole plugged with cyanide" - that wording is close to verbatim if maybe not quite exact. I think the psychiatrist might have been an old family friend who knew the man before the alien encounter.
I read the book around 1980 give or take a few years, but it took a long time for books to reach the small town where I lived so the book could be older than that. The novel I'm trying to remember was not by a well known author. I suspect the precarious finances of the time meant only authors desperate to be published would consent to their books being published in that form. Even though I remember enough specifics for useful Google searching I have found no trace of the novel so I fear it may have disappeared into obscurity (possibly well deserved obscurity from what I remember of it).