Closed Book Meaning

(idiomatic) A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling.

Example:   "Her courteous, sedate, inexorable husband, whose will she could not bend, whom she could not cajole, whose mind was a closed book to her — a book which had lain by her hand for three years, which she had never had the curiosity to open!" — Prisoners by Mary Cholmondeley