In a library, every book is about a different topic, but there's no book on the library's most secret subject. What is it?

Category: Brain · Difficulty: hard

Hint: Consider the nature of the library.

Answer

The library's mission

Explanation

The library cannot have a book about itself, making it the most secret subject.

This is a hard brain riddle — a tricky one that takes a little thought. The answer is The library's mission.

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