I've used Google Books as the main content source for the last two books that I wrote (both of which are freely available for download in PDF): Mille Fabulae et Una: 1001 Aesop's Fables in Latin and Brevissima: 1001 Tiny Latin Poems. To write those books, I relied on literally hundreds of 16th-, 17th-, 18th- and 19th-century Latin books that I found at Google Books. I downloaded copies of the books in PDF and used the Good Reader app on my iPad (a tool I also need to write about someday) to take notes and harvest the content I wanted to use.
Google Books is admittedly a mess, especially for people used to working in academic libraries. The curation of the books is slapdash, with many books completely mislabeled, which makes the search for books something of an adventure - but the thrill of finding just the right book is even more exciting as a result! Working on both of those books, I had more and better access to the books I needed thanks to Google Books than I did when I was a grad student at UC Berkeley which has one of the premiere research libraries in the whole world.
Just as one example, here is a delightful anthology of distich poetry by Barthold Nihus, published in 1642, which was incredibly useful to me. The book is not 12 pages long as the entry seems to indicate; instead, it contains approximately 200 pages of Latin poetry, which means several thousand distich poems. And this was just one of many treasures that I found diligently searching through Google Books, using every trick I could think of to find every volume of distich poetry, no matter how obscure - and neo-Latin poetry is pretty obscure stuff! In fact, writing about this makes me remember how much fun I had writing that book... and it makes me want to write another one just thinking about it. I could easily spend the rest of my life writing books based on the cornucopia of Google Books!