THX, for the documentation. I worked this out myself and indeed the proper amount of tags is the key. It also appeared to me that nookdev-library showed up more tags than I put in with calibre (and see within calibre). A way around is mentioned overhere
http://code.google.com/p/nookdevs/wiki/NookLibraryFAQ just markup your favorite tags see "Show menu keywords" in the linked document.
Besides if you use slashed tags like /fiction and /faction calibre stores ("hidden" feature ? not at all its documented) the files with such a tag (on the device) in the folders "fiction" and "faction", which might make sence if you browse the (external) sdcard with the filemanger.
So yes this method works out reasonable!