Thank you leenia. Mind you, I do have a college report not intended for my eyes describing me as widely read but not deeply!

I don't mind the occasional word I haven't already met, but I do like its meaning to be clear from context. I wouldn't have got through Tolkein, would I? Or geology texts. I can't remember any of the Donaldson words, as I haven't met them anywhere else since. I think in his case it was the result of his autodidactic education, as he was in an isolated situation on a mission station, and may well have taught himself out of books, including Webster. I don't think he knew the words were not going to be recognisable to most readers.
He used them like Homer's epithets, or Viking kennings. As in Odysseus of the many devices. If a writer in English insisted on writing Odysseus polymechan everytime, without access to a lexicon, you wouldn't know whether that word meant longhaired, loudspoken, shortlegged, sharpshooter or whatever. If you could deduce things about Odysseus from elsewhere in the story, you might make a good guess. If the word was used for someone else with similar characteristics, you could build on that. If you knew a bit about language, and could guess the poly was many, and mechan might have something to do with machinery, you could make a stab at it. Wrongly. though for a king he does seem to have been good at DIY. But if you didn't, and he only used that word to describe Odysseus, and Odysseus only, and never described his behaviour in any other way, you'd be sunk. And I was with Donaldson.
Not the only one, either. See this page. Donaldson's wordhoard The word roynish, given no meaning on that page, is the one which led to that passage above. It's something nasty, but exactly what. I have no idea. I did know some of the other words listed, through reading Malory, geology and a few other things. The quoted passage does raise a question about the state of the writer's mind. A bit Coleridge and de Quincey, that.
Another writer who laid linguistic traps for the reader was Dorothy L Sayers, who stuck Greek texts into her novels, which were integral to the plot. And did not provide translations. I can't believe she didn't realise that most people at her time could not even decipher the alphabet.
This is totally different from dealing with writers who coin words, who usually explain what they mean. I remember Andre Norton carefully describing an animal on a different planet. I can't remember the name, or what it looked like, because by the time she had finished describing a small furry herbivorous animal which lived in communities in burrows, and bred rapidly, I was thinking rabbit, and I went on thinking rabbit all the way through the book. The word didn't get in the way of reading, because the meaning was there.

Penny