Comment by Cruel Void on Prinzinc

@Cruel Void

Specific to novels that have a fully different magic system?

Off the top of my head the one that is probably most unusual and doesn't suck terribly is: "I’m Really Not the Evil G**’s Lackey".  It has a lot of refinement with multiple different unusual approaches to a series of different magic systems.

If you want to talk about a magic system that seems different than most but leaning towards a recommendation based on the quality of writing, I'd probably say something along the lines of: "The Beginning After The End".  The writing is just fantastic.  I always judge good writing against "Reincarnated as the Mastermind of the Story" but "The Beginning After The End" is actually even better writing than that standard (it also has a lot more chapters).

On the more popular route... "Tensei Shitara Slime Datta Ken" (I Reincarnated as a Slime") has a decently refined magic system that I'd say is different in a lot of ways that make it good.  Along those same lines I'd put "Overlord" which also has a somewhat unique series of magic systems.  Both of these are a good read with good writing overall.  The anime's for both are amazing although Overlord has some cringe CGI in it that you just gotta ignore (they make up for it with the intro/outro music).  I'm not sure if this has a place to be mentioned here, but I can't help thinking about the above two anime's/mangas without mentioning Arifureta.  The magic systems are also more refined although you don't really get to know them very well except from a distance.  It also has a great anime and the CGI also sucks ba***.  An honorable mention to this list goes to "Is It Wrong to Try to Pick Up Girls in a Dungeon?" which, eh... doesn't maybe have such uniqueness to it as far as magic goes but the approach to the world of magic is somewhat unique.

@Cruel Void For prose, try the Deverry books by Katharine Kerr. Dweomer is based on kabbalistic and Celtic beliefs, among others, but is its own unique system. It's also the series that introduced me to reincarnation as more than do good/reincarnate higher, do bad/reincarnate lower. While reincarnation can result in ties to other people and wyrd (not a typo), it's more "the ties that bind beyond death" then karma.

Edit: Make sure you get the author's edition of Dawnspell and Darkspell, not the editor's edition. Her editor at the time made a lot of changes to them, especially Darkspell, that Kerr was not happy with, so when she got the chance, Kerr reissued them with the story she originally meant them to have. And they're a lot better than the editor version.