Closed book is a term used to describe something that is not open to interpretation or discussion. It is often used in reference to a book, but can also be used to describe any situation where the answer is not open to debate. The crossword solutions for the term closed book are RIDDLE (6 letters), MYSTERY (7 letters), ENIGMA (6 letters), SECRET (6 letters), and CONUNDRUM (9 letters). All of these words refer to something that is difficult to understand or solve, and they all imply a certain level of secrecy or MYSTERY.