I'm still reading the book, but so far, i can tell that it helps a lot in the Lore for the Expansion.
It tells the story and background of Adria, how the Black Soulstone was created, Zoltun Kulle's Past, the new Mortal Life of Tyrael, on Tyrael becoming the "Archangel" of Wisdom and that's as far as i got.
But its as good as the first one, and it creates a foundation for the coming storyline. It's great.
This is actually the kind of thing I miss from the older Blizzard games. I remember when the games came with a manual that was a bloody text book, stuffed full of random lore. Now we get tiny little things.