Synthesized answer
The provided passages do not contain information about what questions remain unanswered in "Essais" by Montaigne.
The passages only offer a title and a brief description of the book, stating it is "The complete essays of Michel de Montaigne, 1533-1592" [1]. There is no content from the essays themselves that would allow for an answer to the question posed.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Essais by Montaigne, Michel de Description: The complete essays of Michel de Montaigne, 1533-1592. --- Google Books --- Title: The Complete Essays by Michel de Montaigne Categories: French essays Pages: 1282