Everything has come easy for Jamie and Garvin up to this point. In less than a day, Jamie acquired magical knowledge that made him go from 0.5 (can't complete successfully the few spells he knows) to 10 (let's say in the order of Thorvill and Urvan) in the mage power scale. I expect an immense price or sacrifice to be paid for this. And the consequence of the 1st lie of impersonating his master hasn't come to fruition yet.
I'm also suspicious of the Flitch character, and his timing of appearing out of the blue in the nether realm when supposedly no other traveler could be chanced upon, and then he imparted Jamie and Garvin tremendous knowledge and power, just before meeting their most dangerous foe. In most stories, the Flitch character is to symbolize you know who, and he expect the most important part of one's being as the trade.