The way I read it, you specify the skill with the bonus at casting time, and you can change at each casting. As all spells that give a bonus with the number of MP spent, it is powerful, but what really limits logician (and makes you wonder if you have it cast or not) is the impossibility to use any rune or passion while the spell is in effect: 'While the spell is in effect, the caster cannot attempt a roll using any Passion or attempt to augment any skill with one' (RQG p396). This is surely a spell that I will not cast with a long duration.
Even if not written, I would limit it to Knowledge skills already known to the spell recipient (i.e. with a skill value over 00%, either as base value or through experience).
Yes, this one, as already shown, is very powerful. It makes you wonder how the lunars didn't manage to take Whitewall earlier.