The Tome of Rubicon is an ancient Dwarven artifact that is buried deep within Sorrow's Furnace. The Tome holds the true name of the Great Destroyer. Ages ago, the Great Dwarf hid the true name of the Great Destroyer in this Tome so that it may never be spoken, as such, no one would know about the true identity of the Great Destroyer.
In the quest Unspeakable, Unknowable, the Stone Summit seek to unearth this Tome and learn the true name of the Great Destroyer to use it for their nefarious purposes. High Priest Alkar enlists the aid of players in stopping them and burying the Tome deep within the ruins of the Stone Basilica.
The symbol on the book is the also the symbol of the Ascalon Vanguard, led by Prince Rurik.
This item's name is probably a reference to the river Rubicon in Italy. In ancient Roman times, a general crossing the river Rubicon accompanied by his army would trigger a civil war since it would be seen as an attempt to capture Rome. This 'point of no return' attribute is probably shared by this tome, since uttering the name of the Great Destroyer would, most likely, trigger something malevolent and irreversible.