Perhaps you should try some spelling variations:
"World general literature series" » "World general literature searches", "World general literature studies", "World general literature sources"
"World medieval literature series" » "World medieval literature searches", "World medieval literature studies", "World medieval literature sources"
world medieval » word medieval, world medical, early medieval
world general » world energy

Try widening your search to All Fields.

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"World general literature series\") OR (\"World medieval literature series\"))".
• Removing quotes may allow a broader search: (( World general literature series ) OR ( World medieval literature series )).
• Adding a wildcard symbol may retrieve word variants: (("World general literature series") OR ("World medieval literature series"))*.