Synthesized answer
The provided passages do not contain information about what questions remain unanswered within the book "Letters for the Advancement of Humanity (1793-1797)". The passage only states the title of a different work, "Early and Miscellaneous Letters by Johann Wolfgang von Goethe," and its page count [440]. Therefore, it is not possible to answer the question.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Early and Miscellaneous Letters by Johann Wolfgang von Goethe Pages: 440