iBooks is just a lame e-reader, Tiziano. iBooks cannot do many, many things that should be obvious for quality e-reader software -- not only this thing you're looking for. It just can't be done in iBooks for now, and I doubt it will be possible anytime soon, given Apple's snail's-pace in improving the app -- if it's being improved at all. (Haven't noticed many iBooks updates recently.)
Just use better e-reader software, and what you're looking for will work like a charm. Here is
Marvin,
www.marvinapp.com ,
http://www.marvinapp.com/whatsnew.html
As you can see from the screenshots, Marvin includes a metadata editor, so you can both adjust your Subjects/Tags in Marvin, and search your Library for specific Subjects.