"The Schools of Arcane Magic - Divination" is a book in Dalaran which can spawn in place of The Schools of Arcane Magic. It can be viewed on the second floor of the Violet Citadel[26.5, 52.2], on the left hand side.
Divination is the school of magic dedicated to gathering information. Powerful divinations can allow the mage to see targets from a great distance, or even view what may normally be invisible. One of the most common uses of divination magic is scrying, which is the art of seeing something that may be far away - perhaps even on another plane of existance.
I'd like to take this time to remind my apprentices that scrying pools are not a toy. They are absolutely not to be used for displaying students of the opposite sex on a pay per view basis. Consider this your last warning.
The legendary archmage known as is perhaps the greatest known master of divination. His potent spells allowed him to peer into the world of Draenor from our Home in Azeroth - and perhaps even beyond. We can only speculate at the true depths of the power that Medivh once held.