Finally there’s a simple, user-friendly way to send content to your Kindle, and it’s directly in the web browser: https://www.amazon.com/sendtokindle
I used to connect my Kindle via cable to my computer and drag over the files manually. While that worked it didn't retain highlights and notes that I made from the book and it didn't sync to my other devices.
Then I used the “Send to Kindle” Mac app for a while. But that didn't support the epub format and more often than not I didn’t get covers for my books.
Let’s just say that it’s been a mess lately to get my books from elsewhere to my Kindle library.
But now I finally have my one-stop-shop for getting content onto my library: https://www.amazon.com/sendtokindle