I have both books and I like them both. The quality of the book (paper, binding, printing, etc) is quite good considering most of the time things like these aren't that great.
As far as content goes, there is a pretty good amount of backstory in it, I believe the book of Cain has more "story" content than the book of Tyrael (part of which is an encyclopedia-like compendium). However both are good reads in my opinion. I particularly liked the section about the creation of the world of Sanctuary as well as the retelling of the events of the first 2 Diablo games.
One other kind of neat thing I found was that the books are a fun source of lore-relevant character names.
They're both pretty cheap on Amazon I would recommend picking them up if you are a fan of Diablo series. If you are into that kind of thing, I would recommend these over say the strategy guide since the guide goes out of date with patches but the lore stays valid. Also the physical quality of the books are better than the guide.
Ruahrc
Great review, thanks. I'll probably pick them up then. I did snag the CE guide for the first one for the art and binding quality (I like my strat guides to be 1. on paper, and 2. hard bound, if possible), but as you say, large parts of it are now out of date. Check that...almost all of it.
I've been a huge fan of Diablo ever since I saw the pre-release posters for the first Diablo at my local Electronics Boutique (RIP). It's consistently been the one series of games (other than Starcraft) I end up spending hundreds of hours on.