Zoltun Kulle was one of the horadrim mages who took part in the hunt for the 3 prime evils in sanctuary. This was during the dark exile when the 3 prime evils were roaming freely in Sanctuary causing mischief.

Zoltun Kulle was in charge of handling the soulstones since that was his specialty. But somehow, Baal managed to damage the amber soul stone during a fight in the desert. So it was Zoltun Kulle who put forward the suggestion that a human heart can be used as an extension conduit of the soul stone to keep baal imprisoned.

Anyways, later after all the prime evils were captured, zoltun went back to the Kehjistan and worked on the black soulstone which is his own creation. It is a vessel that could harness both the essence of a demons and angels.

Apparently he would then use this black soulstone as a substitute for his own empty heart hm.....

Anyway apparently they also may have succeed in killing Zoltun Kulle, so if he were to return, it would be as an undead probably 

1. a worded letter to leah.

2. The Dawn

Anu and the Dragon
This is about how the universe of Diablo series came to be.

The eternal conflict
Explanation about the ongoing battle between angels and demons ..
3. The burning hells

Realms of evil
4. The prime evils

The lords of the burning hells
Info about the 3 prime evils
5. The lesser evils

Info about the lesser evils
6. The high heavens

The angiris council

Some info gleaned from this part. the angel that uses a spear is Imperius, Archangel of Valor. He hates nephalem so he is the one we need to watch out for.Malthael, Archangel of Wisdom went missing after the world stone event. As well as Tyrael.

7. monuments of wonder

The silver city

The crystal Arch

The Diamond Gates
8. Sanctuary the mortal realm

Inarius and the worldstone

According to this, Inarius was a liutenant of Tyrael's. He together with lilith as well as other renegade angels and demons created sanctuary and the nephalem.

The creation of sanctuary

Rise of the nephalem

The purge

This incident is where the rebels were all except Inarius, annihilated by Lillith who was later banished. Also Inarius used the world stone to subdue the nephalem's powers, so successive generations of nephalems grew weak as normal humans as we know them.
9. Ancient history

The birth of civilization

Sanctuary revealed

Basically the vizjerei mage clan was the fakaps who made the demons of hell aware of the existance of sanctuary, because of their meddling with summoning of demons.
10. The sin wars

This was a major event in which the nephalems won over the demons and angels. Lilith removed Ulyssian ( a nephalm's) power inhibitor, which allowed him to reach his full potential. He grew so strong that he single handedly pushed back the forces of heaven and hell away from sanctuary. But he reckoned he was going to fakap, so he committed harakiri.

Anyways the angels had a vote whether to destroy or leave sanctuary alone. Tyrael the deciding vote, voted to not harm sanctuary. Also as part of a peace agreement with the demons of hell, the angels agreed to hand over Inarius to them, for them to torture him and do whatever. Apparently Mephisto has a thing for Inarius, because he flirted and had kids with his daughter lilith. And for that mephisto had a huge grudge against Inarius. This wasn't quite mentioned in this book, but i read it in one of the other diablo novels.

Anyway, Uldyssian remember this name, for he was the most powerful nephalem in history and proof that the angels and demons were right that nephalems could grow to become beings stronger than they.
11. the mage clan wars

There were many mage guilds, but only 3 were major mage guilds. Ennead, Ammuit and the notorious Vizjerei. Anyway the other 2 guilds don't like the Vizjerei, because it was this one guild that got them in trouble that led to the sin war. So anyways the Vizjerei were up to their shenanigans, so these other 2 guilds went on a crusade to wipe them out. They almost succeeded, until the Vizjerei summoned the demons. But then Horazon stopped his brother bartuc by killing him so that sort of ended the war.

Anyway the aftermath of the war, mages fell out of favor with society. They then look toward religion for societies guiding pillars of power.
12. The dark exile

The lesser evils headed by Azmodan and belial, raised the forces of hell in rebelion against the prime evils. The prime evils decimated 1/3 of the forces of hell before they were defeated and banished to sanctuary.

In sanctuary they caused all manner of mischief, while azmodan and belial fought each other in hell, for who would be the reigning king of hell.
13. The hunt for the three

Tyrael created the horadrim faction secret society of mages. These mages came from random mage guilds

Anyways tyrael gave them soul stones to trap the prim evils, which they managed to do.

14. The hunt for zoltun Kulle

In the whole book i think this was the most note worthy, because it mentions the dark soul stone, what it is, and what it can do. And also who created it and for what purpose. In the Diablo 3 trailer, you could see the dark soul stone so this should be hugely relevant for D3.
15. The rise of Zakarum

with the decline of mage guilds influence over society, the biggest religion aka the order of Zakarum, held control of politics all across the world of sanctuary.
16. Modern history
The darkening of tristram
The dark wanderer
This whole part of the book is about D1, D2 events which most of you already know. The new highlight though was that the dark wanderer was actually Aidan, King leoric's son. And that Leah is the daughter of Aidan and Adria the witch.
17. Sanctuary: lands and cultures

This part of the book talks about stuff post D2 after the destruction of the world Stone after the landscape had changed drastically e.g. mount areat blew up and the area became known as the dreadlands etc etc...
The deserts of Aranoch

The dreadlands

ensteig

ivgorod

kehjistan

khanduras

scosglen

the skovos isles

The torajan jungles

west march

xiansai
18. a letter to leah

It was written just after Cain was hit in the face by a meteor and survived. I'm guessing he wrote this after he was saved by the hero, or while he was still lurking around inside the monastery after the meteor fell.

Anyway it's heavily hinting that Cain will be passing on his duties as last of the horadrim mages, to leah.

19. the last page has a map of Diablo 3 sanctuary.