Masque of the Red Death had three products during its (un)life:
Boxed Set - Had the basic rules-alterations for the setting, plus 3 adventures: Red Jack (PCs vs. Jack the Ripper), Red Tide (PCs vs. Dracula) and Red Death (PCs vs. ...guess who?).
Gothic Earth Gazeteer - Hundreds of pages mixing real-world VIPs (Mark Twain, Bram Stoker, Winston Churchill) and fictional characters (Sherlock Holmes, Abraham Van Helsing) as if they all existed alongside each other. Also detailed several Secret Societies, including a mysterious gipsy tribe called "Vistani".
Guide to Transylvania - Researched to (un)death, filled to the brim with flavor and campaign possibilities. Included stats for Van Helsing, Johnathan Harker, Vlad Tepes, Mihnea (Dracula's son), among others.