mystery ***
enigma
secret ***
closed book

"I didn't mean to open pages in a closed book."

But your own past can be a closed book, even at fourteen.

They all looked at the closed book, and then at one another.

History is never a closed book or a final verdict.

So I think that's pretty much a closed book for me.

What ever happened to a reader who could say to a closed book: "Tell me a story!"

Students should understand that science is rarely a closed book.

He was himself, a closed book to others, as always.

Up to now, the subject of his father has been a closed book, at least publicly.

It's not a closed book if he starts against Denver.