Complex sentence: David "Zeb" Cook is a game designer best known for his over 15 years working at TSR, Inc. on Role-playing game design, particularly his lead in the work on Advanced Dungeons & Dragons 2nd Edition and Planescape campaign setting.
Split into simpler sentences: David "Zeb" Cook is a game designer best known for his work at TSR, Inc., where he was employed for over fifteen years. He is particularly known for his lead in the work on the "Advanced Dungeons & Dragons" 2nd Edition roleplaying game and Planescape campaign setting.