A lot of books can very often be freely accessed online with the use of a currently functioning website mirror of the Russian libgen project. For example, the domain name libgen.is works for me right now, but this can depend on the country from which you’re trying to access the website from, as it may be blocked in many countries. Try searching with Google, Yahoo, DuckDuckGo or some other search engine of your choice to find a mirror of the project which works for you.
Another great way of accessing books and many articles online is to have access to the website of the arg project, which is currently located at the domain name aaaaarg.fail. But you will need to register a personal account there to get to the works themselves, and the creation of an account requires an invitation from an already existing member of the project. If you don’t have an account there yet and need one, you can try to contact me and give me your personal email address, and I can send you one there.
A lot of articles can often be freely accessed online with the use of a currently functioning sci-hub project website mirror (for example the domain name sci-hub.do works for me at this moment), by first, if you’re using the Google Chrome browser or a derivative of it, installing the extension required for the functioning of sci-hub, finding the correct DOI number of the text in question and inserting it into the main ‘search field’ of the sci-hub website. Try searching with a search engine to find the exact DOI number of your desired text, and the mirror of sci-hub which works for you, as some portals may be blocked in certain countries.
Also don’t forget to check the archive.org project if a copy of the work you need is by any chance uploaded there, if you don’t have the luck and don’t find your desired copy of the book in any of the previous solutions. There are also ways to obtain a book from this particular project by borrowing the book in question with an account on the website and then removing the DRM protection via some software, like using the anti-DRM plugin for Calibre, but I won’t go into so much detail about that here.
There is also the option of becoming a member of the Facebook group that is currently called Ask for PDFs from People with Institutional Access, where you can put up personal requests for a certain book which may not be available anywhere else and if you’re lucky someone will respond to provide you with the text in question.
Finally there are various so-called torrent trackers dedicated to sharing books, which are specific websites sharing torrent files and so-called magnet links, but I personally don’t know much of those dedicated to books, except for maybe bibliotik, but unfortunately I don’t even have personal access to that one, simply due to the fact that I haven’t been using the torrent network to download anything for a very long time.
There are also probably other some torrent sites dedicated to books in existence which I personally might not know about. But when using the torrent network to download anything these days you also have to be quite careful about which specific client you actually use for downloading itself, for example if you’re using the Windows operating system, you should probably find a copy of the very old 2.2.1 release version of the uTorrent client from a trusted source, since the newer versions of this particular client apparently have various nasty things written into the code of the client software, or perhaps try an alternative like qBittorrent.
Of course, don’t forget that if you personally have a digital copy of some specific book or article which you think others might make use of, you’ve always got the option of uploading and thus sharing that work in any of the mentioned sites above. I’ve even noticed some authors upload their own writing to these kinds of websites in order to promote their own work in this way and thus have it distributed more widely.
There may be a lot of other online databases, peer-to-peer networks, or ways of obtaining various texts online for free which I may personally not be aware of. In that case you can contact me and I can add that particular solution to this list.