Apologies for being offtopic. I have noticed that the link "z-lib. is" has been declared a scam site on the official zLibrary reddit (https://www.reddit.com/...). Although they have a clear internet adress (https://singlelogin.re/), the reqirement of an account, the personal domain management and the occasional instability make them quite unfit for the average lurker looking for a book. Also stated in their thread, I recommend the use of https://annas-archive.org/ as an easy to use substituent for "z-lib. is".