Additional materials for use with The Lost City:
The Lost City Campaign Sourcebook has design history and notes, campaign suggestions and reviews.
The Dungeon Master's Guide to Cynidicea offers a fair bit of expanded information that would help develop this module into a campaign.
Dragon #315 has an article and map (for version 3.5) expanding the module entitled "Mystara: Return to the Lost City"(pp. 88-92). The article is difficult to utilize alongside the module, however, because in the article Zargon has a Challenge Rating of 16 and the difference between D&D (Basic) and version 3.5 is significant.
Elder Evils has a chapter on Zargon. His stats are as presented in Dragon #315 (above). The temple tier in Elder Evils is presented as Zargon's lair; the rest of the pyramid and underground city are only briefly mentioned.
Dungeon #142 has an adventure (for version 3.5) entitled "Masque of Dreams" (pp. 14-30) that once again pits the adventurers against the Zargon cult.
Classic Modules Today B4: The Lost City and Classic Conversion: B4 The Lost City are designed to assist in converting the module for use with D&D 5e rules.
Iconic Encounters: B4 - The Lost City
is a set of screen-resolution maps for use with virtual tabletops, etc.
Note that the first iteration of PDF scans sold by Wizards of the Coast omitted the 4-page pullout black & white map section (Tier 6, Tier 7, Tier 8, Tier 9, Tier 10, Elevation, Underground City Map) that was included in the print module. Later PDF iterations included these missing maps. These interior maps should not be confused with the blue & white maps on the module's cover/folder interior (Tier 1, Tier 2, Tier 3, Tier 4, Tier 5, key).