Sideloading is putting a book file on your Kindle without buying it from the Amazon store.
Yes. In fact, it is supported by Amazon! You use Amazon's own free software called "Send to Kindle" to do it.
Sideloading allows you to buy books at much lower costs. It also ensures that you keep your books, no matter what Amazon does to you. In the past, Amazon has banned individual Kindle books, and even wiped the entire Kindle libraries of users. Learn about that here: https://www.zdnet.com/article/why-amazon-is-within-its-rights-to-remove-access-to-your-kindle-books/
1. PC users, go here: https://www.amazon.com/sendtokindle/pc
Mac users, go here: https://www.amazon.com/sendtokindle/mac
2. Install and log in. If logging in doesn't work, use this method instead: https://www.amazon.com/sendtokindle/email
3, Download and the unzip the free test file below (it contains the first few pages of The Equation for Excellence: How to Make Your Child Excel at Math.)
4. Upload the file onto your Kindle. That's it!
Stick with it; it's worth it. Learning how to do this will let you save hundreds, if not thousands of dollars. Here are things to try:
1. If you're having trouble logging into the Send to Kindle app, go here for some advice: https://www.amazonforum.com/s/question/0D54P00006zSyaYSAS/send-to-kindle-sign-in-does-not-work
Amazon needs to make Kindles able to read sideloaded content in order to advertise that feature. It's a major selling point for them. However, they'd obviously prefer you buy from their Kindle store. Stay persistent: here are some tips.
2. If the Send to Kindle app doesn't work, try using the email option instead.
3. You can contact Amazon support here: https://www.amazon.com/hz/contact-us/foresight/hubgateway-csp
Learning to sideload means big discounts on e-books. It also means that you can get many older books for free! Most classics are available, for free, at Project Gutenberg. Learning to sideload means you can get them all free on your Kindle.
The other great news: while sometimes initial setup can be a pain, once it works, it usually keeps working fine!