You only get one brief chance to enter Kistandalius' back room, if you pay the poet standing just outside the shop to recite some poetry. The mage comes out to listen, and gets distracted just long enough for you to grab the spellbook off the table, and unlock the chest to get a (potentially quite good) dagger. The orb is just decorative, and you can not move or interact with it.
Unlike most shopkeepers, Kistandalius can detect you trying to enter the back room even if he is out of sight (chased out or moved with the mage's Swap Places special skill), and can teleport you away. If you missed your chance with the poet, the only other way to get past is defenses is to kill him (he sells charms and spell books, though, so killing him for a random spell book and dagger isn't really worth it, unless you do it just before heading to the wastelands).
