It seems I had a project of collecting book websites, which was abandoned, now revived since I got pissed off with Libgen and Anna's archive got zilched (for me anyway). Some better than others.

OceanofPDF
https://oceanofpdf.com/
This is a new one. So-so.

Memory of the world
https://library.memoryoftheworld.org/

Liber3
https://liber3.eth.limo/

Reference book sources that Google hides

Google is so powerful that it "hides" other search systems from us. We just don't know the existence of most of them. Meanwhile, there are still a huge number of excellent searchers in the world who specialize in books, science, other smart information. I suggest using DuckDuckGo .. it works for me .

www.refseek.com - Academic Resource Search. More than a billion sources: encyclopaedia, monographies, magazines.

www.worldcat.org - a search for the contents of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need.

https://link.springer.com - access to more than 10 million scientific documents: books, articles, research protocols.

www.bioline.org.br is a library of scientific bioscience journals published in developing countries.

http://repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related science.

www.science.gov is an American state search engine on 2200+ scientific sites. More than 200 million articles are indexed.

www.pdfdrive.com is the largest website for free download of books in PDF format. Claiming over 225 million names.

www.base-search.net is one of the most powerful researches on academic studies texts. More than 100 million scientific documents, 70% of them are free

PS. Worthwhile checking some websites from Rask's list in https://open-slum.org/. Some of them work for me.