Perhaps you should try some spelling variations:
"authors, german / 18th century / correspondence" » "author, german / 18th century / correspondence", "authors, germany / 18th century / correspondence"
"authors, german / 18th century / corresponding" » "author, german / 18th century / corresponding", "authors, germany / 18th century / corresponding"

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"authors, german / 18th century / correspondence\") OR (\"authors, german / 18th century / corresponding\"))".
• Removing quotes may allow a broader search: (( authors, german / 18th century / correspondence ) OR ( authors, german / 18th century / corresponding )).
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