I keep thinking about this because I've lately been reading Blood Meridian by Cormac McCarthy, and it's just ridiculous. I have to look up 1-2 words per sentence sometimes, something I'm only used to doing for Joyce. Apparently McCarthy is well known for doing this sort of thing.
The book was written in the mid-1980s -- it's modern. But the terms used therein belong to another time, and are mostly unknown to modern English.
How do authors pick up such a broad vocabulary of words that they can effectively disguise themselves as a hundred years older than they are? These words can't be used in day-to-day speech; nobody would understand you. If you can't use them, how do you remember them?
Examples from Blood Meridian: rebozo, shellalegh, hackamore, osnaburg, bungstarter, weskit, jacal, farrier, escopeta, caesura. Not a single one of these words have I ever heard reference to anywhere else in my entire tour of existence.