Perhaps you should try some spelling variations:
gael robert » gal robert, ebel robert, jaef roberto
robert j » robert _, robert e
a robust » _ robust
robust e » robust _, robert e
heitmann » heimann, heinemann, hermann
book » ebook
books » ebooks
ebookss » eebookss
ebooks » eebooks

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: (( gael robert j. heitmann (book OR books) ) OR ( a robust e. heitmann (ebookss OR ebooks) ))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( gael robert j. heitmann (book OR books) ) or ( a robust e. heitmann (ebookss or ebooks) ))~".
• Adding a wildcard symbol may retrieve word variants: (( gael robert j. heitmann (book OR books) ) or ( a robust e. heitmann (ebookss or ebooks) ))*.