Not that Diablo has ever been known as the pinnacle of literary achievement for its tie-in products, but this is somewhat poor even by those standards. The artwork is beautiful, the layout is serviceable, and the binding is great. The book itself is nice to hold and to read, though I liked the more rough-hewn texture of the Book of Cain more by comparison. This one's shiny/smooth exterior and gilded pages make sense given the supposed origin, however, so it attempts to be true to the fiction in its design, which is a nice touch.
The only problem is the writing.
Cain's fictional voice has always been subject to gross hyperbole. Never have we expected Shakespeare from the man, and his descriptions of otherworldly beings have always tended toward the absurd. Nearly everything is "God forbid this should ever happen" or "My mind can scarcely comprehend this blasphemy", to the point where if taken at his word, the guy would have been a gibbering mess long before the events of Diablo 2. Unfortunately, the writer's stylistic missteps are much worse here, for other and, sadly, more damning reasons. Given the fact that Cain is supposed to be a scholar of some renown, it's hard to take the work seriously, because the writing is anything but scholarly in tone. In fact, it almost all comes across as the opinion of someone with rather too much confidence in his own analysis, and he constantly qualifies things with "It is my belief" or "In my view" (I paraphrase) even when those things would reasonably be considered, were they true, either common knowledge, or one of very few options of opinion on the subject, rending such qualifying of his opinion as a wanton waste of text. Worse, it reads contrary to the way the character has generally presented himself in the past because it takes one minor stylistic point and overdoes it by several orders of magnitude.
Little problems like this abound, and anyone with a discerning eye will find that this mars the readability of the text to the point where they may just stop reading and start skimming instead, pausing for pictures. If you find no fault with Diablo 3's mishandled script (such as Cain calling Leah "brilliant" for suggesting that the Stranger's sight of a fishing village during his fall must be Wortham—you know, the fishing village right next door? What insight!), you'll probably find this to be no more than a step or two down from that. There is, sadly, no writing of the quality we saw with the game's follower characters, who arguably added more life to the tale (along with the player characters themselves) than any of the major figures.
Tyrael's sections sound like Tyrael did in Diablo 3, and thus get the job done with a good-hearted mediocrity. Cain's pieces are the big problem, and if you were a big fan of the Book of Cain, you'll find the voice here to be a flimsy replica of that tome. The writing there was hardly stellar, but certainly of a higher caliber, and the distinct lack of Cain's at this point well-established voice hurts the Book of Tyrael immensely, because it reads like what it actually is: poor writing failing to give us something believably canonical—in tone, if not in content.
The information seems to be accurate enough, and as I said above, it really is a wonderful book in other regards. It's a shame someone with considerably more editing experience wasn't given the time to work on it. Reading through almost any given page is enough to show that this needed a healthy rewrite before being anywhere near ready for serious publication.
Given much of the work that has come before it, however, and the quality of writing that the target demographic is willing to tolerate, this is only a marginal notch down from business as usual for Diablo. Most people will honestly never notice or care, and if you're on the fence about this particular purchase (it certainly isn't cheap), use previous work as a general guideline for your expectations. If you weren't offended by the ghastly writing in the novels, for instance, you'll find nothing to put you off here either; and the book is entirely worth owning in that case, as it remains a high quality collectible piece in its physical dimensions and illustrations. If all you're looking for is basic lore information and beautiful artwork, you could do much worse than the Book of Tyrael. While I'm greatly disappointed in the overall quality of the intellectual product, I'm not sorry that the physical one is sitting on my shelf.
Given the limitations of Amazon's review system, I can't give this 2.5 stars, so it will have to stand at 2, as I can't in good conscience bump it up to 3. Another editing pass, though, and this easily could have been a 4-star product.