This is a sub quest of A Crusade to Faydwer in the Swords of Destiny quest line.
It begins in New Tunaria by inspecting a underwater chest at -194.97, -10.66, -920.28 You must "Quench the thirst of Devotion".
This is done by obtaining a ewer at either -226, 36, -1011 or -227, 36.49, -706 These areas are opposite ends of a sublevel in front of the Hall of Truth. Inspecting the Ewer will start a sub quest to fill it "The Capture of the Living Flow" and is repeatable until you have enough to satisfy the thirst. Make certain each time you collect a new Ewer to inspect it and initiate the subquest so that you get proper credit for the main quest.
To actually fill the ewers, you will need to kill water spirits, level 55^ mobs in the large pools in the front of New Tunaria. Updates to fill the ewers are random, and you will need 8 full ewers total by the time you are done, so prepare to be there a while, and group with as many folks as you can.
After you fill a ewer, you will need to empty it into the Urn of Devotion. -125, 41.36, -777.02. This is a large ewer located in an indentation on the side of the Devotion tower. To get to it, you must perform a skillfull fall off of the bridge to the tower, and onto the walkway that rings it. The Urn is on the side that faces North. You will need to empty four filled ewers into the Urn to satisfy the thirst of Devotion.
Now it's time to satisfy the thirst of Purity, and you will need to repeat the steps you just performed, except you will be dumping the filled ewers into the Urn of Purity (-130.32, 40.51, -957.61) Which is located on the tower of Purity, below the main level (another tricky jump onto the sub level again) and facing South, toward the Tower of Devotion. Pour four ewers into the Urn to fill it. Then check the underwater chest to unlock it, collect the book inside, finish the quest, and update "A Crusade to Faydweer"