Perhaps you were thinking of the Integral trees, but your mind was in Xanth...
I have read them, quite good, in fact, I believe I've read just about everything from Niven, Pournelle, Asimov, Heinlein....I do have a problem with some of the newer authors tho', sometimes it seems they fill thier novels with fluff so they can do a trilogy when one or two books would have gotten the point across..of course, they gotta eat too.
Thanks all,
Mike