I used to download Project Gutenberg books to my computer at work to read back in the days when I worked for a workaholic boss who wanted everyone to look busy all the time. I was a secretary. All the work I had to do was what was given to me, so when none was given . . . It worked like a charm except once when another secretary asked me what I was working on that caused all the letters on the screen to be green. I said that if I told her, I'd have to shoot her. She apologized like mad, and I just laughed.

If you Google free downloadable books, you will get good answers. Presently while my public library is closed to in-person visits, I, now retired, am borrowing e-books on line books and spend most of my time reading,

I too have used Project Gutenberg which is great. There used to be daily free kindle books offered on amazon, not sure if they do that any more. I have never heard of Z library. I do use my library for kindle borrowing probably about 99% of my reading is from the library for free ( which never was totally closed).

Further agreement about Project Gutenberg. Other sites I’ve used include Girlebooks and the link is a list of more resources.

https://reedsy.com/discovery/blog/free-books-online