Tagging is very interesting. I searched "Delicious" for Hawaii. I got travel information, as well as a Hawaiian newspaper. I then added travel to my search to refine it. I sometimes find with tagging, I get some unusual hits, some of which are good, that I may not have thought of for a tag. I like the idea of free form tagging.
Delicious could be useful to keep booklists. These could be divided by genre. We would have to agree on tags, so that everyone would be able to find "our" lists. This could also be useful for craft projects. It could be tagged by ages, etc.
I did a search for "series “and found a nice site of juvenile series and sequels. (www.mcpl.lib.mo.us/readers/series/juv/).