Many already know that Adobe offers free ebooks as sample downloads for the Adobe Digital editions program. (They have books in English, German and other languages.)
I just discovered that some of the books are available on a temporary basis.
So, it might be worthwhile to re-visit on occasion.
It may never be as large as
http://gutenberg.org but it might have that useful book you need.
The Adobe selection:
https://www.adobe.com/solutions/eboo...k-library.html