Related Clues:

• Carolyn Keene's genre
• See 16 Down
• Something to figure out
• It's puzzling
• Challenging puzzle
• Whodunit, for example
• It's a mystery
• Inscrutable sort
• "Hmm" provoker
• It's no no-brainer
• Appropriately named monthly of the National Puzzlers' League, with 'The'
• WWII encryption machine
• Closed book
• Something of a mystery
• Something mysterious or puzzling