The British Library uploaded an app last week that is really worth getting. They dug into their archive of out of print public domain titles and bundled a thousand of them into this app. The app is free.
You have to download the ebooks separately, of course, but you can also sample them before you download them. The titles in question are based on high resolution scans of 18th and 19th century reference works. The selection leans heavily to history and science, with some fiction and poetry.
These are scans and have not been converted to text. That might be a problem for some but I prefer it. I've had too many bad experiences with Google's automated system.