Tomes of Knowledge Quest/Spoiler

Requiments

 * The New Frontier Quest Completed. Otherwise Cael won't accept tomes.
 * Some tomes can be found during quests.

First Tome
Go to the orc fortress located northwest of Zao and enter the fortress, when you are underground go north and west.

Second Tome
You can access this tome during the Uneasy Alliance Quest (mission 2).

Third Tome
Completing mission 2 of Uneasy Alliance Quest.

Fourth Tome
During mission 7 of The New Frontier Quest.