I have just finished Thorns (1967) by Robert Silverberg.
By the way, I'm glad I read this after his Downward to the Earth, although the latter was, I believe, published two years later. As I wrote a few days ago, there was a point in that book where I nearly gave up; I'm glad I didn't. If I'd read Thorns first, though, I might have given up there and then, and on this writer as a whole, which would have been a mistake.
I ought to start with the positives. Robert Siverberg's writing carried me along and I found the two principal characters quite sympathetically and truthfully portrayed; their stories take centre stage most of the time, which is just as well.
The conceipt that drives the book is silly (and unnecessary, if truth be told). I was going to say profoundly silly, but that simply isn't an adequate description. The silliness is fractal: from which ever angle you approach it, and whether you look at it as a whole or burrow down into its detail, it's silly. Mind-numbingly so. It's a testament to Robert Silverberg's writing that you are able to forget it for long stretches and actually have the bad luck to reach the ludicrous final chapter. I shall have to read more of this writer's work, but profoundly hope that none of it is as silly as this book was.
(As an aside, someone in the Publishing area of the Chrons said that she did not like too many POV switches; she wouldn't like chapter four where, for effect, two completely separate events alternate faster and faster until they occupy alternate sentences. And it's not as if either thread was meant to be exciting, which is just as well.)