By free books I mean PDF copies of old, rare, and out of print books that are out of copyright, and even recent books. Now, there is no substitute for an actual book that you can hold in your hands. I have thousands of them. However, sometimes it is not possible to find a copy of a book that you can afford or the book is simply not available anywhere. There are also occasions when you just may want to check something in a book but don’t want to have to buy it just to do that.
For books out of copyright, my first choice to obtain a PDF is Google Books and my second choice is Internet Archive. They have many of the same books, but sometimes you will only be able to download a PDF from one site. If no PDF is available for download and you just want to check something in a book, you can sometimes search for the information you are looking for and view the page it is on. For some books that are still in copyright, Internet Archive will allow you to digitally check out the book to read it online or look up what you need in it. You can sometimes find downloadable PDFs of recent books at Z-Library. This is great when you need to look up something in an expensive book that you would never buy.