Perhaps you should try some spelling variations:
"nijhoff book titles 20" » "nijhoff boston titles 20", "nijhoff op titles 20", "nijhoff book entitled 20", "nijhoff book intitled 20", "nijhoff book titles 60", "nijhoff book titles 2.9", "nijhoff book titles _"
"nijhoff ebook title 20" » "nijhoff book title 20", "nijhoff ebook title 2", "nijhoff ebook title 10", "nijhoff ebook title 21"
"nijhoff ebook title 26" » "nijhoff book title 26", "nijhoff ebook title 2", "nijhoff ebook title 21", "nijhoff ebook title 25"

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• If you are trying to use Boolean operators, they must be ALL CAPS: (((("nijhoff ebook title 20") OR ("nijhoff book titles 20"))) OR ("nijhoff ebook title 26")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"nijhoff ebook title 20\") OR (\"nijhoff book titles 20\"))) or (\"nijhoff ebook title 26\"))".
• Removing quotes may allow a broader search: (((( nijhoff ebook title 20 ) OR ( nijhoff book titles 20 ))) or ( nijhoff ebook title 26 )).
• Adding a wildcard symbol may retrieve word variants: (((("nijhoff ebook title 20") OR ("nijhoff book titles 20"))) or ("nijhoff ebook title 26"))*.