It would seem that some authors have taken to spelling magic with a "k" at the end, which is magick. Why are they doing this? Is there a substantive difference?
Crowley used this spelling if I remember correctly. So if someine is actually basing his magic system off his in a Urban fantasy or something it might make a bit of sense.
In alternate fantasy worlds? Not really.
I've seen it work, but generally I think it just sounds pretentious. It gives a certain fantasy punk feel to things if done well, but in general I prefer to stick with the common spelling.
It's not just a fantasy novel thing; a lot of New Agers and neopagans spell it that way too.