I run a small charity project that self publishes children's books (as in, books by children). They are via Amazon's print-on-demand service and always produced the same way. We do no external advertising but various schools and authors might tweet the Amazon link.
About 20% of our books appear on Google books. Here's the list
Some of the books in Google Books have a thumbnail, some don't.
My question is: what triggers one of these books to be listed in Google Books and why does the cover design only come along occasionally?