Frank Spriggs was a very able man but something of a closed book.

弗兰克· 斯普里格斯是个非常能干的人，却不被人所知晓。

《柯林斯英汉双解大词典》

Fishing is a closed book to me.

我对钓鱼是一窍不通。

youdao

A closed book.

一本合上的书。

youdao

And other classes are completely closed book. You can't bring anything into it, even a calculator, so.

而有些课的考试是完全闭卷考试。考试时什么都不能带，甚至计算器都不行。

考试应该这么准备 - SpeakingMax英语口语达人

Who can possibly remain a closed book, when others try to open it and leaf through?

NEWYORKER: Hack Work

But diplomats and other foreign observers admit that the army is a closed book to outsiders.

ECONOMIST: By-elections in Myanmar

This election may have closed the book on a Vietnam veteran ever being elected president.

FORBES: Magazine Article