When I’m looking for an ebook, I check 6 sources:

Library Genesis - https://libgen.is/

Z-Library - https://singlelogin.re/

VK - https://vk.com/docs

RuTracker - http://rutracker.org/forum/index.php

Twirpx - https://www.twirpx.org/

Flibusta - http://flibusta.is/a

If the book is in English and it is not in these 6 sources, you should look at the Internet Archive. There are a lot of digitized books there due to the cooperation with American libraries:

https://archive.org/

The book can only be available for rental for an hour. The script helps here.

Look Internet Archive.

If the book is not in all of these sources, last chance is googling it. If there is no book in Google, then most likely the book was never digitized. You can digitize it yourself. Look Book digitization.