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Matt recently revealed the covers for two new books in the series that will be published in September this year.
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Escape from Castle Ravenloft is especially noteworthy because it is the only book so far to be associated with a D&D setting other than the Forgotten Realms. All the books in the series have tie ins to current D&D Adventure Sourcebooks. The Mad Mage's Academy is tied to Waterdeep: Dungeon of the Mad Mage for instance.
More Isle of Dread material from WotC would make fans of BECMI D&D and fans of Mystara happy. The setting has been teased in numerous D&D5E books, but Isle of Dread is the only book to fully embrace the setting.
See also: An old analysis of mine about the old Endless Quest Books and possible connections to Mystara (Piazza thread)
-Havard