Originally Posted by Blossom
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.
Thanks for the advice Blossom. I managed to get some of my non Amazon books into my archive and I can read them in the Kindle app on my phone and tablet and they keep synchronised.
However if I go to my laptop and open the Kindle Cloud Reader I only find the books from Amazon in the Cloud. Shouldn't I be able to view my added books from any Cloud Reader running in a browser without having to authorize each PC?