@eschwartz: I can't remember why I didn't decide to go with this solution back then (I think I was afraid it would be more difficult to update Calibre on a regular basis. Not sure I was right - any thoughts on that?).
In any case, I'm going to give it a try now as it may be exactly what I need.
@fidvo: somehow I was convinced that you can have only one library folder inside of the Calibre Portable, i.e. 'Calibre Library'
. Glad to have been wrong! Storing the library outside of Calibre Portable is more convenient (e.g. when I would copy the whole installation just to experiment with some settings/versions) but it's not a must.
@BetterRed: thanks for your tip. I would have never thought about that as stupidly enough, I was pretty sure you can have only one library inside of the Calibre Portable folder