I have completed several of the tomes. They are just quests like any other, and usually level 10-20. Nothing special unfortunatly, I had hoped for more as well. In my experience the Ancient Ring is the nicest reward found with the scrying stones and I have used probably 250 stones.
My advise is to find a place where the mobs are easy and plentiful before you accept the quest. For example, if you need to kill 20 undead, go to the island in the NW corner of Antonica or go to Forest Ruins in Qeynos, makes it a lot easier to finish.
Hope this helps.
Edited, Sat Jul 2 13:55:16 2005 by FealixTheCat