I first want to say, I didn't intend on writing a review for these books. But between just finishing the third book, the impending release of the next game they are based on, and the small number of reviews up here, I figured I should review it to give some more feedback to fans of the game on whether this is worth their time and money. I would also like to say I am a big fan of Blizzard's games, and have read now all the Diablo and Starcraft books. I'm going to copy and paste this review across all three books, but don't worry, I won't give you any spoilers (not that there is much to give).
To get right down to it, the books are based on a man named Uldyssian, his brother Mendeln, and a woman named Serenthia and their fight for freedom for their world basically from angels and demons. The angels and demons have set up followings in the world of Sanctuary to recruit worshipers. It sounds interesting, the execution was poor. Uldyssian and Serenthia basically have a magic power that allows them to do whatever they want, and all they have to do is think it hard enough. Mendeln is pretty much the first necromancer and while his magic isn't the, "just think it hard enough" type, he just magically has the right words pop out of his mouth to get the job done with very little actual training. The ties into the actual games themselves are pretty slim and as a stand alone to someone who hasn't played the games, the books don't hold up well and they add hardly anything to the lore of the universe itself.
The first book is decent, but slow. The second book is slow and slightly less decent. The third book is slow, and the ending is atrocious. I can't say much more without spoiling the ending unfortunately. As others have mentioned, Knaak says on basically every other page that this person isn't their name, but the son of, the brother of, the daughter of. It gets really old, really fast. You never at anytime think the characters are in any real trouble for their lives and this also applies to the atrocious ending the the books, which I promised not to spoil. There is no character growth hardly at all except that they devolve slightly as their powers grow stronger, but it is never expanded on or anything. For some reason I can't understand Knaak and Chris Metzen (who is basically lore master at Blizzard and approves everything that goes into all Blizzard books) seem to go out of their way to reverse typical thinking and make the angels out to be worse than the demons. They did something similar in the Warcraft universe where they went out of their way to make the orcs less evil, except they didn't do it nearly as well. Maybe they thought they were being original, all they ended up being was boring.
Really that is all I can comfortably say without spoiling anything, but otherwise, I strongly recommend avoiding these books to everyone, even the hardest of the hardcore Diablo fans who want to know everything about the universe the game is set in. What makes these books even more disappointing is how well Richard Knaak did the other books in the Diablo universe and how good my friend said he did with the books in the Warcraft universe.