As is seemingly the trend, media distributors put DRM (Digital Right Management) onto the files in a vain attempt to stop them from being copied. This interferes with the ability to Ubuntu to simply read these files without going through some hoops.
Here is how to do it:
Go to Amazon Website->Digital Library->Manage Kindle->Select Book for Transfer via PC
Save book file (xxxx.azw) to /tmp
sudo apt-get install calibre
Obtain "DRM" removing software (Author Page):
Extract -> /tools/Calibre_Plugins/K4MobiDeDRMxx.zip to /tmp
Open Calibre (go through wizard if youd like):
Applications-> Office -> Calibre
Preferences->Plugins->(Browse for file Button)
Select: File Plugins
Enter your Kindle Serial Number (Found in settings menu)
Select Calibre->Import Book
Select your download book (xxxx.azw) file
Title should appear in your list of books
Right-click on the book title and select "view"
Enjoy your DRM free book on Ubuntu!