Complex sentence: The game, based on WEG's earlier Ghostbusters RPG, established much of the groundwork of what later became the Star Wars Expanded Universe, and their sourcebooks are still frequently cited by Star Wars fans as reference material.
Split into simpler sentences: The game, based on WEG's earlier Ghostbusters RPG, established much of the groundwork of what later became the Star Wars Expanded Universe. West End Games sourcebooks are still frequently cited by Star Wars fans as reference material.