Quizzes are popular on The Internets, right? Which Food Mixing Device Are You, What Is Your Thread Count, all those things. I never take them, but people seem to like finding out which fantasy author they are. (I know which fantasy author I am. I checked.)
What I think, though, is that with long series of books it should work in reverse. Rather than "Which Vorkosigan Novel Are You?", where you tell it you like hard sharp cheeses and things that are pale lavender and it tells you you're Diplomatic Immunity, you say you like Diplomatic Immunity best and it tells you that you should watch for falling objects and job opportunities this week, or take a chance on love, or do not forget to turn off the oven when you leave the house. Sure, people who were born roughly the same time of year have some things in common, but people who like Pigeon Post best probably have several more things in common. (Among other things, they're nothing like those Coot Club losers, honestly.) And you can imagine the conversations: "My kids just don't get along. Well, my son is a Nine Tailors and my daughter is an Unpleasantness at the Bellona Club." "Oh, yes, I have a cousin who's a Nine, and once they get a notion in their head, hoo boy!" And then you'd read down the column in the morning and go, "Honey, you're a Yendi, right? You're supposed to take some time to yourself this week. And drink plenty of water."
(This is why they don't let me write horoscopes or advice columns. Like every third thing would be, "Make sure you're getting good protein. If you're not allergic, toasted hazelnuts are nice." Which does not appear to be the sort of thing people want.)
Also this would be great for still-living, still-writing authors: "I thought I was a Taltos, but now I know I'm an Issola! It's so much more accurate now!"
What I can't figure out is how many books a series really needs for this to work. The Narnia books, for example, do not seem quite numerous enough to me, although maybe it's that I'm incapable of conceiving that anybody might like The Last Battle best and so I'm only counting six.
The other problem with this theory is that it's giving me the urge to go reread all the Aubrey & Maturin novels to figure out what my O'Brian sign is.
And the other other problem is that some authors seem clear to me that they do not require a series for this to work--Diana Wynne Jones, for example--and others really do seem like it'd be best with a series. Possibly this is just because I want to exclude The Documents in the Case and am not sure anything is clearer if you have both Perelandra and Prince Caspian as options.
If you like, you can tell me which one in a series is your favorite, and I will give you your literary horoscope for the week. It might involve hazelnuts, though. I can't promise anything on that front.