I've found some interesting stuff through;

Library of Congress e.g. Rickenbacker's Fighting the Flying Circus.

and Freebook Sifter

I've also installed a couple of apps, mainly to browse and discover Gutenberg: Goot eBooks and GuteBooks Free Ebooks. One find using the first of the aforementioned apps was a book called Green Balls; The Adventures of a Night Bomber, by Paul Bewsher.

So, mostly WWI stuff.

If you wish to keep and maintain an Elibrary, or bookshelf, I recommend checking out Calibre

After an apprenticeship of several years, I now think I grasp a few of the intricacies and the flexibility of this very powerful management software. Why should you use it? Backup. That's why.