Internet Archive: Digital Library of Free & Borrowable Books (Movies, Music & Wayback Machine)
https://archive.org/
Bushra Abdul Hamza Abbas is correct that Library Genesis has more books than Amazon.com at least the last time my grad students crawled the two sites in 2014. This is logical because Library Genesis stocks many out-of-print or copy-free books as well. However, the idea of a shared virtual library has been considered by the Hague court of law, US courts and it is not an easy subject to follow
Chapter Copyright Infringement of Digital Libraries and Private Inte...
having said that you will have to find Library Genisis as while fairly stable and distributed over many platforms and websites it does change locations every two to five years. https://libgen.is/ https://libgen.unblockall.org/ https://libgen.io/ and so on.
Origin story: like many European researchers in the 1990s the prohibitive cost of reading many necessary articles from Springer, Taylor and Francis, etc. created many private shared libraries in University networks. If your chemistry department paid for a paper then it was stored on hard drives accessible by universities in the network. However, this practice began much earlier in the countries within the Soviet Union. Plus genetics was according to Lenin just a false science or capitalist tool. So they coded well and Library Genisis became the most popular pirate academic website. True, to place a book on Library Genesis you have to say that the copy was purchase by you or your institution. Anton Vrdoljak is right that the Internet Archive theoretically is truly open source, but a quick scan in 2018 found copyright materials in a minimum of 10% of recently uploaded books. This is very similar to audiobooks on youtube. Harry Potter audiobooks are continuously uploaded by fans.
The search engine https://filepursuit.com/ has an app that is distributed by Andriod and Apple which clouds the legal waters considerably. Files not webpages are found, and those files are heavily books but include recent TV-series, movies, etc.
Notably, a sort of internet education clubs have been in existence for decades like https://sanet.st/ where members put books et al, on temporary storage sites like rapidgator, nitroflare.
I hope that helps
C.G. Yukna
PS Genesis, Pirate Bay and similar sites claimed in the 2000s that they had sold a majority of the hard copy books because people used their PDF, epub, mobi files only to browse as in a bookstore not to read
Thanks @ B.A.Abbas, A Verdoljak and C.G.Yukna.
for such a nice discussion to promot real education.
If any other sites there to mention will be highly appreciated.
Legal Sources
National Academy Press
Has a ton of reference books like recommended dietary intakes or Agricultural Development most as they affect the United States
Here is a quick way to search NAP copy the URL and add the subject you want after a space
https://www.nap.edu/search/?term= search query
All the downloads are free and legal. You do have to create a free account and give a valid email.
I have used them for quite a few books on the immune system that were excellent
The site: operator and Online Libraries like
the National Library of Congress
write in the question field at google: site:https://www.loc.gov/ keyword
or National Library of Medicine
example of a query in duckduckgo
site:https://www.nlm.nih.gov/ + subject
Many countries have wonderful National Libraries Britain and France come to mind and have books available and these sites are not always well indexed by search engines
CG YUKNA
Here is the link, you may try to get the free books and publications https://scihubtw.tw/
TERMCAT offers Cercaterm, an online consultation service, available from the Center's website www.termcat.cat, with free access for all users.