This book appears in your inventory when you find all 10 Azdaran fragments in the northwest mines of Keldagrim, which can only be mined after completing Birthright of the Dwarves.
You need level 95 Mining to collect all of the fragments. Some require lower Mining levels than others.
Transcript
This is the last resolution of the Council of the Elder Magi of Keldagrim. May the Sleeping God forgive us when he wakes.
The dark god Zamorak has corrupted the magic that Guthix placed in our blood, such that all dwarves will slowly transform into twisted abominations. Saradomin has protected his Imcando dwarves from the curse, but our god is sleeping and cannot help us. The city is already falling as chaos dwarves run amok in the streets.
Our greatest wizards have found a remedy, but at a terrible cost: all Keldagrim dwarves will permanently lose their ability to work magic. The curse will remain dormant in our blood, but - starved of magical power - it will not take effect.
The sudden end of dwarven magic will throw our city into chaos almost as dire as that which Zamorak hopes to cause. Without magical farms, we will starve; without magical lights, we will be blind. No band of petty councilors can preserve this city through its darkest days - only a king. We will therefore use the last of our authority to install Captain Gozdaron as absolute ruler, and pray that he will quickly win the inevitable civil war and bring order to the city.
Recorded by Archmage Azdaran, Head of the Council of the Elder Magi of Keldagrim. May the sleeping God forgive us when he wakes.