Hello! It has come to my attention that not many people know how to get a hold of free ebooks. My personal ethos is to buy the book if the author is still alive, even if you do it after you read it online.
But, nonetheless, here are some great sites!
1. Zlibrary
My personal favourite truly! I have so many books from here and they aren't just in English. There's a great variety and even a recommendation system. Making an account is very easy and you can enjoy books in pdf, epub, rtf and a few other formats. The text is very clear and the whole site is user friendly. It does have a 10 books a day downloadinit if you're bot buying premium but I find it very reasonable.
2. Archive.org
It works just like a library and you can borrow books as epubs for up to two weeks with the possibility of extending it. Sometimes you might have to queue but again, they are great quality. I have not used it as much as I'm the type to stick to one for good but it's still very useful. If I remember well, it also has audios and videos.
3. Gutenberg project
Also a very useful site that allows you to download books in pdf. I found that it has a great search engine even if the interface might seem a tad outdated. It's relatively simple to use and can ensure fast access. From what I know, there's isn't a daily download limit.
4. Library genesis
A very simple if not rudimentary interface, I am in love with they search engine and how quickly it can get to exactly what you want by ISBN.
5. 101book.ru
I use this more for books in Romanian but it can be quite useful as well, although it has a few ads if I remember well.
Feel free to add what other sites you use!
Hope this helps!