Don't know if there's been major changes to the Library structure in Windows 8, but in Windows 7, Kindle For PC creates a folder called My Kindle Content in the Documents folder (C:\Users\Home\Documents). That's where it puts the books, which also tend to have really weird names instead of proper titles -- for example B00A1N2F5Y_EBOK.azw instead of "Deadly Magic (A Grace Holliday Cosy Mystery)". So try looking for anything called My Kindle Content on your system and your books will be in that.
Remember to first pay a visit to Apprentice Alf if you want to back up anything that has DRM, which many Amazon ebooks are often infested with. With the latest version of his Tools and an updated Calibre, you should be good to go.
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