Apropos of nothing, my friends have suggested Xanth to me on and off again for over a decade now. I know nothing about it except that Amazon will deliver it to my doorstep. What three books should I be buying?
Perhaps not the one I mentioned, since I just spoiled it for you. :) I have found that adults don't tend to like Piers Anthony's writing (it's filled with puns and adolescent soft-porn humor), but if you want to better know where your friends - who might have read Xanth books at ages at which they could better appreciate the writing style - are coming from, I would pick the first three of the series:
* A Spell for Chameleon
* The Source of Magic
* Castle Roogna
If you want some more mature books from Piers Anthony, the Apprentice Adept series and Incarnations of Immortality series are better than the Xanth ones. Plus, they're both only 7-8 books long, so it won't take you forever to read them all (if you actually get into them).
Apprentice Adept is kind of a cool dual-world series, with sci-fi in one world and fantasy in the other, and the ability for some to move between them. You should start with the first in the series, of course: Split Infinity.
Incarnations of Immortality also has the juxtaposition of sci-fi and fantasy, but in one world. The "hook" for this series is that the major "forces" in the world (time/death/fate/war/etc...) are actually offices which people fill (think The Santa Clause, but without Tim Allen or christmas). The first book of the series is On A Pale Horse.
A final word of warning: I read both of these series a long time ago, so it could be that they're actually not as mature as I remember =)
A la The Source of Magic. One day, all of the magic in Xanth was turned off. For awhile, Xanth was like Mundania. http://en.wikipedia.org/wiki/The_Source_of_Magic