I think magic should be very low key in the UU. Stephen Strange (jnr) isn't very good at what he's doing, and he probably uses all manner of eastern hallucinogens to achieve his goals, this will play a large part in the ambiguity of magic.
I'm a firm believer that a good mystery should not be ruined by solutions, by having a magic based book we will be forced o examine how it all works, and should this happen we shall undoubtedly find an unsatisfactory solution.
For example, you say a spell, who is the spell to, who made up the words, how is contact made, where did those one is saying the spell to come from.. a whole host of problems arise.
This said, something more 'spiritual' or religious is something that could be explored, but this will probably appeal to only a few and I only suggest it because it's something that i have personal interest in.