Demonsbane was the first book released in 2000 that contains Diablo lore. It was written by Robert B. Marks and was released in an e-book format, but unfortunately did not have a good amount of sales despite heavy marketing on the Blizzard website.
Demonsbane was primarily written to give flavor to the series, defining the world of Sanctuary as a living and detailed world. It talks about the battle between the angels and demons, and how it effects Sanctuary when they decide to go for it. The story is seen from a warrior named Siggard's perspective, who was the only survivor in the battle against the demonic invasion of Entsteig. He remembers nothing of his final battle, and only carnage and revenge is in his mind. As he hunts for the demons that killed his friends, he starts remembering pieces of the battle that ensued and finds that his nightmare has only just begun.
Some argue that Demonsbane is not canon (lore endorsed by Blizzard), however Blizzard has not disowned this book in any way and has even included it in a later release. There is no way to confirm this theory however because none of the characters appearing in the book are included in the Diablo games.
As mentioned before, Demonsbane was released in an e-book format, however it was also included in the later released Diablo Archive, which is a paperback novel consisting of four different Diablo books.
Reviews of this book can be found in the following sites:
The following is an excerpt from the first chapter of the book:
Siggard startled awake, the sounds of battle still ringing in his ears, as though he had just been in the midst of the bloodshed.
Exhausted, he lay on the bank of a road, the trees on both sides obscured by a light mist illuminated by moonlight. He tried to sit up, only to have his back explode in pain. For a moment he rubbed the sore muscles and kidneys, and then he struggled to his knees.
Blinking, he wondered where he was and how he had gotten there. The road did not look familiar at all, and there were no visible landmarks. He scratched his head, trying to think, and winced for a moment when his fingernails ran over a tender spot.
Siggard was a large man, well grown, with a full brown beard. But now his usually placid gray eyes were haggard and his beard was in a tangle. He shook his head; he knew he had been at the field of Blackmarch, a shield-man in the army of Earl Edgewulf. And they had been fighting someone, but who he could not say.
Groaning, Siggard gained his feet. He would first have to find his way to the battlefield and try to rejoin the army, but what he truly wished was to rejoin his family in Bear's Hill. That would have to wait until the fighting was done, though.
Taking stock of his gear, he noticed his sword was rather more notched than the last time he remembered, and his leather jerkin and trousers were ragged but intact. Where his coat of mail had gotten to, he had no idea. His wide shield was also missing.
Cloaked in a mist drawn eerie in the moonlight, Siggard tried to get his bearings, but no matter which way he turned, he couldn't tell where Blackmarch might lie. Finally, he picked a direction and began walking.
How long he walked before he reached the gallows, Siggard could not say, though it seemed hours. Regardless, he found himself facing a fork in the road. To one side of the road there was a three-way sign, but it was too dark to read it. On the other side stood a gibbet, a decaying corpse dangling from it by a worn hemp rope.
Unbidden, the words of one of his comrades in arms came back to him. "Hanged men have angry souls, you know," old Banagar had said. "That's why they hoist them at crossroads. That way they can't find their way back for vengeance." Banagar had always been rather morbid, he reflected.
You can get the original e-book format of Demonsbane from here:
To get the paperback version that also contains 3 other Diablo novels, you can get this one:
Given the very cheap price of these novels, I can recommend them to any one even partially interested in the Diablo games. Have fun!