Do remember that most new books are under copyright law, so it's illegal to get books under copyright from the internet for free.
However, if you want to read university level books, they can be extremely expensive. But since you want to learn, you can easily get older editions for much less on Amazon. Most newer editions have the same content as the old one, the only difference is that the questions and exercises are rearranged and changed, and prettier/colour pictures added to be able to charge students with ridiculous sums of money. I would prefer a hardcopy of a book so that I don't strain my eyes and can easily read them without my computer lagging.
pdfdrive.com and archive.org have some university level books if they are too expensive for you.