They let you follow series of books, see what order they come in, track which ones you've read, and be notified of new books in the series as they come out.
I've been doing this in a text file on my computer with the help of Wikipedia. It will be interesting to see if doing it on the web makes it easier to manage.
I'm firecatstef over there, if anyone wants to be my "contact."
Ultimately it would be nice if Goodreads offered this feature, since that's where I do most of my book tracking and reviewing. (Although I haven't updated it in a while.) I'm firecat over there: http://goodreads.com/firecat
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