Here’s a moderately important literary first that also happens to be on the Pringle Modern Fantasy 100 list.
Updike, John. The Witches of Eastwick. Franklin Press, 1984. First edition hardback, a limited edition signed by Updike (which precedes the trade edition), a Fine copy in decorated leather boards, sans dust jacket, as issued. Pringle, Modern Fantasy 100 91. Bought off eBay for $19.99.
The day Updike died, at lunch I spotted the cheapest Fine copy online (which I think was something like $40) and got the dealer to agree to hold it. By the time I got home from work (this being back before I owned an iPhone, and thus no access to my home email), the dealer had already sold it to someone else. So I bided my time until an even cheaper copy presented itself, which it finally did.
I suspect the fact I got this so cheaply is a sign of the general price decline of hypermodern literary firsts in general and Updike in particular. A few months ago, Heritage Auctions sold off someone’s Updike collection, and I don’t think the prices most things went for were particularly high.