Synthesized answer
The provided passages do not contain information about what questions remain unanswered in the book "On Light and Other High Frequency Phenomena." The passages only include the title and a description of when and where the lecture was delivered [1]. Therefore, I cannot answer your question about unanswered questions.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA by Nikola Tesla Description: A lecture delivered before the Franklin Institute, Philadelphia, February 1893, and before the National Electric Light Association, St. Louis, March 1893.