Cryomancer
Arcane
This is a very questionable logic. What exactly can high nobility offer to the magicians that the magicians can't make or take for themselves, if they are powerful enough? And if they are weak, what use is their magic anyway. I mean, in principle, if on any given world, magic is powerful - then there's no chance that world isn't already ruled by mages, whether secretly or overtly. And if magic is weak, or protection from magic is readily available - then it's largely worthless, or at least not nearly enough for any kind of conquest.
Books? And mostly arcane books? Books was rare on medieval times. And using your logic, no mercenary army would ever work for a noble, since they can take the gold of the noble...
Actually, nobility had power because it was the warrior class. A world with magic and mind control would make the warrior class much weaker.
Nobility will have access to arcane arts in most situations where magic is powerful and accessible.