The phrase "closed books" typically means something that is difficult to understand or figure out. Antonyms for this phrase could be "open books," meaning something that is easy to understand or transparent. Other antonyms could be "reveal," "uncover" or "expose," indicating that the information in question was once hidden or inaccessible, but has since been made available. Alternatively, one could use antonyms such as "solved," "understood," or "unraveled," meaning that a difficult problem has been resolved or a complex concept has been explained. In any case, antonyms for "closed books" all suggest a process of uncovering or revealing information that was previously concealed or difficult to grasp.