Try this
https://archive.org/ they have some new books but mainly they have scans of 10s of thousands of old books. The scans are available in many formats and do cover periods like WWII. And yes it is above board.
Amazon also offer some free ebooks for their Kindle, if you look at the Kindle best sellers list they have a free tab which changes on a regular basis so you can get some good books every so often.
Cheers
Richard