Perhaps you should try some spelling variations:
"nijhoff book title 20" » "nijhoff boston title 20", "nijhoff op title 20", "nijhoff book titles 20", "nijhoff book table 20", "nijhoff work title 20", "nijhoff book title 2", "nijhoff book title 10", "nijhoff book title 26"
"nijhoff ebook title 21" » "nijhoff book titles 21", "nijhoff ebook titles 21", "nijhoff ebook table 21", "nijhoff ework title 21", "nijhoff ebook title 1", "nijhoff ebook title 2", "nijhoff ebook title _"
"nijhoff ebook title 25" » "nijhoff book titles 25", "nijhoff ebook titles 25", "nijhoff ebook table 25", "nijhoff ework title 25", "nijhoff ebook title 2", "nijhoff ebook title 5", "nijhoff ebook title 26"

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"nijhoff book title 20\") OR (\"nijhoff ebook title 21\"))) OR (\"nijhoff ebook title 25\"))".
• Removing quotes may allow a broader search: (((( nijhoff book title 20 ) OR ( nijhoff ebook title 21 ))) OR ( nijhoff ebook title 25 )).
• Adding a wildcard symbol may retrieve word variants: (((("nijhoff book title 20") OR ("nijhoff ebook title 21"))) OR ("nijhoff ebook title 25"))*.