Site: libgen[dot]is for probably 99% of all books with a pdf that exists online. However this site often gets blackholed by most institution’s DNS/firewalls because they don’t want you using it obviously. Can bypass using a VPN or TORbrowser.
Generally when I do any pirate stuff via browser only I just use TORbrowser. Yes it’s not endpoint secure but it just needs to be enough so my ISP doesn’t complain. The safest option is always to purchase a trusted VPN which will usually get you through any institutional firewall as it’s an “insulated pipe” that encrypts all traffic inside it.