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(1967) Eighth in the Three Investigators series, and the weakest so far. The prince of a tiny European monarchy invites the boys abroad to attend his coronation. The US government gets involved, and employs the boys as "junior agents" inside the castle. It's less a mystery than a Ruritanian adventure, and feels out of place in the series, though I probably liked it fine when I was in the target audience.
This was my #DoubleSpin for January