The phrase "closed book" is often used to describe someone or something that is difficult to understand or figure out. Luckily, there are many synonyms that can be used in place of "closed book" to convey a similar meaning. Some of these synonyms include enigma, mystery, puzzle, conundrum, riddle, and quandary. Each of these words suggests something that is challenging to decipher and requires careful consideration or investigation. Using these synonyms can help you add variety and nuance to your writing or conversation, and can help you more precisely convey the idea of something that is difficult to understand.