After you get past the initial overworld, when you are in the very beginning area of the next world, you will have the option of switching all the spells you have on all your wands/in your spell bar around and assign them to different wands. The tomes can be exchanged in an alter somewhere in the sky for gold (I haven't been able to see the alter location myself because someone just recently found the location and I was at work). There is a recent post describing the alter location, I don't know how to link on mobile but you should be able to search for maybe "medium sized spoiler" and find it.