I bought a Kindle for a family member too. She also has limited funds to spend on books so one of the first things I did was show her how to download public domain books. There are two methods. You can use the Kindle browser to visit the mobile versions of the websites and click the download links just like you would on your computer. Project Gutenberg, Feedbooks and MobileRead also have books that are basically comprised of links that you click and it downloads without having to use the browser. You can find out more at this thread link.
https://www.mobileread.com/forums/showthread.php?t=92311
On the other hand, I bought my mom a Sony 505. She is computer savvy, and she wanted to borrow books from her local e-library.