Whizbang Dustyboots
Gnometown Hero
Van Richten's Guides would be my suggestion, most of which have some interesting ideas that can easily be adapted to your game of choice.
The question for me is: did anyone buy and use Masque of the Red Death back in the day, and how was it?
Is there anything good you'd recommend buying?
if I can only grab, say, 3, what 3 should I get?
Out of all the Ravenloft books I owned from 1E-3E The ones that stick out for me we're the 2E books, Domains of Dread (the hardcover - TSR 2174), Carnival (TSR 11382) and Van Richten's Monster Hunter's Compendium Volume's One-Three (TSR 11447, 11507, 11613) immediately come to mind as ones that were good and wished I'd kept.The 2e Von Richen Guides were pretty revolutionary at the time because they treated monsters as characters in their own right, fully customizable and diverse.
Over the last 25 years I did 3 great purges of my RPG collection, I'm talking about 15-20 boxes. Every time a conversation like this comes up, I always regret getting rid of a good portion of what I did, even though I'd never read or use them again, I still miss them for nostalgias sake.Boy do I wish I still owned my original I6 module.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.