As you make your way through the Battlefields of Eternity, look around for piles of scrolls called Angelic Scrolls. These contain parts of The History of Pandemonium.
0:00 - The History of Pandemonium, Part 1
0:30 - The History of Pandemonium, Part 2
1:01 - The History of Pandemonium, Part 3
1:28 - The History of Pandemonium, Part 4
Also within the Battlefields of Eternity, look for the dead bodies of blue-colored winged-demon corpses that resemble the Messenger of Azmodan corpses from Act III. These, however, are called Ancient Corpses, and they contain the parts of The Discovery of Sanctuary. They are absurdly rare - I had better luck finding them in adventure-mode games, rather than campaign-mode games.
1:55 - The Discovery of Sanctuary, Part 1
2:19 - The Discovery of Sanctuary, Part 2
3:02 - The Discovery of Sanctuary, Part 3