Kistandalius: Pay the minstrel outside the shop to recite some poetry; the mage will come out, giving you a short, one-time-only chance to grab the spell book off the table in the back room and get the knife from the chest. Save first. You may also be able to use Telekinesis, perhaps with Wizard's Sight. Do not toss a teleporter stone in the back room; you will not be able to get it back.
The Alchemist: Open the door, the mage will close it. Open it again, toss a teleporter stone in and close the door. Then teleport in.
