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.
The Carnival is discussed in Van Richten's and briefly discusses how it's originally from the Feywild and switched places with a Ravenloft carnival, which is the one in Wild Beyond the Witchlight.Carnival is Carnival that travels through the Domains IIRC. It's pretty unique and easily dropped into any campaign. I never played or fully read the 5E adventure Wild Beyond the Witchlight but from what I did read of it I think one could draw some parallels between the two books at parts.
Fraternity of Shadows my belovedDomains of Dread,
Van Richens Guide to Ghost - good advice on turning ghosts into full mystery adventures
Night of the Walking Dead is a great adventure on its own and the start of the Great Conjunction Saga metaplot (Feast of Goblyns, Ship of Horror, Roots of Evil)
Castle Forlorn - a great use of time shifting in a dungeon
Bleak House Adventure is pretty cool too
Note that Fraternity of Shadows use to have a lot of fanmade material including advice on how to 'improve' adventures, but I havent checked it for a couple of years now
Their take on Meredoth, in their Nocturnal Sea Gazetteer, made him one of my favorite darklords.Fraternity of Shadows my belovedand absolutely seconding VRGT Ghosts

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