Ah... hi guys! Sorry I never saw your answers before. I think I never got notified about it.
Anyway thanks for your answers. In the meantime I got the extension working. If you like you can find it here
https://github.com/dalanicolai/gnome-tracker-extension. Despite its name, it is actually a quite versatile deep file search extension and it includes a cb keyword to search directly in the calibre database file.
@kovidgoyal Thanks for creating calibre and share it. It is a truly awesome software obviously! (Just want to launch some books quickly sometimes without opening the calibre gui first)