Thanks to all of you.
The changes to the shortcuts I made yesterday didn't work just because I wrote them wrong. Silly me
Today I upgraded calibre to the last version (from 0.6.16 to 0.6.17) and the new shortcuts (with the changes) don't work anymore.
Until today, the path has been like this:
C:\Programs\Calibre\calibre.exe --with-library X:\[calibre IT] or [calibre EN]
C:\Programs\Calibre - E-book Management\calibre.exe --with-library X:\[calibre IT]
but Windows says that "C:\Programs\Calibre
is not a valid path". Is this just a problem with the spaces in the new installation folder?
By the way: switching libraries from the preferences is a neat solution :P