The Light and How to Swing It

The Light and How to Swing It is a rare and precious book written by Sire Uther the Lightbringer, it was never intended to be read by anyone other than the one for whom it was written, for Arthas Menethil.

But over time, many people have benefited from Uther's wisdom and experience. The book was retrieved from the Athenaeum in Dire Maul, and was until recently in the hands of Lorekeeper Mykos. Following the Fourth War, she graciously donated the book to the Paladin archives at the Sanctum of Light, and removed the mark from the Athenaeum, allowing all paladins to browse through it.

It was also a quest starting item that leads to the paladin version of the Royal Seal of Eldre'Thalas.

Source
The Light and How to Swing It drops from all bosses in Dire Maul, and can be found inside of Dusty Tomes in Dire Maul.

As a quest objective

 * The Light and How To Swing It