Lee Sopwith was telling me how he’d been using Google for his genealogy research and it had turned up a book that was (partly) online and was very useful to him. There’s a lot out there but it’s generally not organized for genealogy. Here’s what little I know:
Google Books. This is not specifically genealogy books; it’s any and all books Google’s managed to get ahold of and scan. Some of these are the full text, while others are just snippets. Generally they’re full text if the copyright has expired, and snippets otherwise. Thomas MacEntee has details in an older article titled “Google Books for Genealogy.” Everything here is indexed in the regular Google index.
Genealogy at the Internet Archive: This is genealogy-related books only, and the full text is available for everything in the collection. And they’re all indexed. But not by Google; as far as I know, you have to go to this site and use their search mechanism. Lisa Louis Cooke has a more recent article with details and links.
For both of these, the joy & the sorrow is you may find something invaluable — or nothing at all.