Perhaps you should try some spelling variations:
a robust » _ robust
robust b » robust _
robert b » robert _, robert e, robert j
heitmann » heinemann, hermann, hartmann
heimann » hermann, heinemann
book » books
ebook » ebooks
ebooks » ebookss, books
heilmann » heinemann, hermann, hellman

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

• If you are trying to use Boolean operators, they must be ALL CAPS: (( a robust b. (heitmann OR heimann) (book OR ebook) ) OR ( gal robert b. heilmann ebooks ))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( a robust b. (heitmann OR heimann) (book or ebook) ) or ( gal robert b. heilmann ebooks ))~".
• Adding a wildcard symbol may retrieve word variants: (( a robust b. (heitmann OR heimann) (book or ebook) ) or ( gal robert b. heilmann ebooks ))*.