Originally Posted by Lady Fitzgerald
... If ever you do need a library with a flat structure to use outside of calibre, you can use calibre's Save to Disk function to generate one for you.
Just to add, Save to Disk either works on the entire library, or just those books you have currently selected. It also has the option of only exporting one format (.mobi for example)
If you are on Windows, I use a handy program called xplorer2 which has a command "Browse Flat" which will flatten the Calibre folder structure (in memory) and let you browse it that way.