"Mantissa" means essentially an unnecessary verbal addendum. Mildly amusing, mildly erotic, mildly neurotic. It mostly seems like the work of a dirty old man treading water, mildly undecided between putting sex or love, or some combination of the two, at the sole apex of life, while suspecting those same impulses for trapping him in boring dialogues and marriages. I thought his suggestion to this imaginary woman that she try working as a reviewer was ugly and uncalled for.
Fowles had unaccountably been in England too long, after several early years believing he was somehow Greek. But we're still treated to a lot of nonelucidated namedropping of Greek words, Greek authors, etc., etc. Per my, verbally, far more interesting book, "Are the English Human?", written by a Flemish/French immigrant to the UK in the 1920s, Fowles has become thoroughly English here; he has become indifferent to the English language, although he's still enthralled by a long Greek trainwreck of a word or two. It seems Brits of a certain age can write these unobtrusive, non-explicit stage plays, one after another, if they're less honest than Fowles.
Happily, the "meta" part of this meta-novel seems largely subdued. Just as happily, the hilarious and pointed asides about deconstructionists, postmodernists and other weirdos of 80s academia who "proved" that authors don't write their own books are barbs now missing their target, since I can see no evidence that readers bother to even buy postmodernist books.
Suspicion #1 confirmed: this was the last novel Fowles wrote
Suspicion #2 uncomfirmed: he was only middle-aged, not old
Suspicion #3 unnecessary: Was he (happily?) married at the time?