When I first installed the Kindle application on my PC, I couldn't figure out how to get my Kindle format books that weren't purchased from Amazon into the software. There isn't anything in the menus or the help files that explains the process.
It actually is very easy.
- Install the Kindle App.
- Find the book you want to open (these typically have a ".mobi" extension)
- Double-click the book and it will open automatically in the Kindle app and as long as you don't move it, the Kindle App will keep it in the library.