Originally Posted by JMikeD
I don't think books will sync between devices unless you buy them from Amazon and they are in your Kindle Archive (on Amazon's servers).
If that's what you're asking.
I believe there may be a way around this for books not bought from Amazon by using Calibre, but I'm a bit unsure of the exact details.
There is a feature is called the Personal Documents Cloud for that and you get 5GB of space to upload books or docs to. They become part of your Archive and will sync between devices. There is no need to sideload them using USB anymore. You can email or use Send to Kindle from your PC to upload them to the Cloud then access them on any Kindle or Kindle app.
Just go to your Manage Your Kindle page on Amazon's Site and set up your Personal Documents Email address then you can email the books to your Kindle App and they will sync on all Kindles and Kindle apps.