Second paragraph of third section (there are no chapters):

Chnassian astronomers are, of course, faced with a similar situation vis-à-vis our own galaxy, but their mental conditioning being somewhat different from our own they appear to find no great difficulty in accepting the notion that the cosmos and everything within it is one vast sensorial illusion. Thus a Chnassian astro-physicist having counted the number of Cepheid variables in a particular nebula would automatically assume that his result was ‘wrong’ simply because his eyes told him it was ‘right’. Similarly nothing is easier for the Chnassian to accept than concepts like ‘negative mass’, ‘reversed time’ or ‘black holes in the universe’—incidentally, their own term for these latter phenomena is ‘oscitations’. One famous Chnassian museum contains a large room in which a working model of the entire cosmos is said to exist. No one has ever seen it, but countless thousands of Chnassians have heard it. It consists of a quiet chuckle endlessly repeated in total darkness. Its texture, though admittedly indescribable, is rumoured to resemble that of a Gruyère cheese.

A short sf novel from 1974 about a middle-aged lecturer in an unsatisfactory marriage who distarcts himself by science fiction and flirting with a student. You can get it here. Both activities suddenly get more serious as the world that he is writing about turns out to be real and distant, and intruding on Earth, and the student starts to flirt back. The story ends with restoration of the status quo rather than any change to the frame of reference. It's about halfway between Kingsley Amis and Douglas Adams, and I was so struck by a certain similarity of tone with Hitch-hiker that I wrote and asked Christopher Priest, who knew both Cowper (John Middleton Murry Jr) and Adams personally, if he thought that one had partly inspired the other. (He said no.)

This was the shortest book acquired in 2014 that I had not yet read. Next on that pile is Cloud on Silver by John Christopher.