Several years ago, I downloaded a number of PDF files of really old geological publications from Google Books. I did a search a few days ago and this function seems to have disappeared from the Google site. Anyone know if this sort of thing is still available, or has it been deactivated?
In the righthand upper corner of the search page for GB there is a little "hexagonal" button where you can chose advanced search and once there also select "only readable" and/or books that are "downloadable" (I don't know what english terms GB uses as everything is defaulted nowadays so I see only the swedish interface)
Anyhow...once on the results page you have found a book that you like to look at open it in a new window (that way you have your result page intact) Now you can look at your book. If you want to download it - again go to the "hexagonal" button on the right and click it - now a dropdown list will have the option "download pdf" - klick it and follow the instructions.
I'd also suggest you to explore archive.org, many of the books that are not downloadable directly from GB are available as pdfs from archive.org, and as it often turns out these books are actually GB sources. The lesson is that the system is not perfectly logic and my experience is that the more you play around the more you'll find, and you can find A LOT!