This article explains how to load books onto your Kindle Fire from outside the Kindle Store. Instructions apply to all Amazon Fire tablets.
Amazon Kindle: Transferring Books to Your Kindle
If you buy legal, non-DRM protected eBooks from other sellers, you can easily transfer those books to your device. There are also websites that offer free eBooks that you can download to your device using the Amazon Silk browser. You can find the files you download in the Docs app.
Tap a file to open it. If it is in the standard MOBI for Kindle Fire format, the book will open in the Kindle reader app.
You can transfer files to and from your Kindle Fire as if it were an external hard drive, using a USB cable:
