The book drops like all Legendary Books, random sentient mobs > 39 iirc. I got mine from the Auction House.
First 4 pages drop from Morroval (very low drop rate 1-25 or less) in Duvaire (Ram Duath, Angmr)
Last 4 pages drop from Snowreap Goblins in Bruinen Source East, Misty Mountains.