If I understand correctly, only the books out of copyright (1929?) are available for download. The ones with downloads availble have a download link at the top right hand corner. Be sure to get the ePUB format for the nook.
You can also check the Gutenberg project website, primarily because they have ePUB (and a host of other formats) available (definitely preferred over PDF on the nook). Google only has ePUB and PDF (for the few I checked). I'm not sure if Google books has ePUB for all their books but Gutenberg definitely does.