In the Win 8 app the books appear to be stored in some in app database or something (that was the speculation anyway). Last I heard no one had found an actual location for them.
In the regular Kindle for PC (which works on Win 8) they're stored in Documents in a folder called My Kindle Content.
EDIT: A bit more info. I just saw a post on Amazon that mentions files are stored at...
	Quote:
	
	
		| C:\Users\<username>\AppData\Local\Packages\AMZNMob  ileLLC.KindleforWindows8_RandomCharacters\LocalSta  te\cache | 
	
 ...but, they're stored in multiple fragments not like they're stored in the regular Windows for PC.