In case you missed it, switching.social has a search page for public domain eBooks:
https://switching.social/public-domain-ebooks-search/
It uses a custom DuckDuckGo search for ebooks in Gutenberg, Adelaide University, Wikisource, Gutenberg Australia, Gutenberg Canada, Faded Page, Standard eBooks, Runeberg.org, Lonnrot.net and dbnl.org
If you know of any other sources of PD eBooks, especially non-English sources, let me know.