The Arcane Tome is found in the Arcane Sanctuary, very near The Summoner. Once a character has defeated that mad SuperUnique, they can click on the tome to open a red portal to the legendary Canyon of the Magi. The book tells about the history of Tal Rasha and the Horadrim's ancient battle against the three prime evils:
Seekers of the Tomb of Tal Rasha will find it through the Portal. But know that the glowing glyphs recorded here in my Arcane Sanctuary are the signs of the six False Tombs.
The missing Seventh Sign marks the Tomb of Tal Rasha... Of the Horadrim he might be called the foremost.
It was a shining - but brief - moment for the Mage Clans when they set aside their differences and worked together against the common enemy.
The Horadrim relentlessly pursued the Three across the desolate Empires of the East, and even into the uncharted lands of the West, leaving the Archangel Tyrael's hands unblemished.
Presuming the Three to be vanquished, the Horadrim's unstable fellowship began to dissipate. Abandoning their sacred charge to safeguard the three Soulstones, the disparate Mage Clans began to squabble amongst each other over petty differences.
Their conflicts not only dissolved their brotherhood, but strengthened the Evils which they had buried beneath the cold earth.