Ok I attended BlizzCon 2011 and there was a booth with this book there for preorder. They also had it there so you could see the book. The Book of Cain is written from the perspective of Deckard Cain, the last Horadrim Sage who appears in all three games. The book itself is a mixture of lore and art. It opens and ends with a letter, 'written' by Cain to Leah, the girl we've seen so much of in the previews and trailers. The rest of the book details the creation of existence and the making of Sanctuary, as well as pieces of lore we've heard only a bit about, the book fleshes out those little whisperings. It also covers a brief run-down of the games, leading up to the upcoming game. After the lore pieces, it goes through a few monsters, the Prime Evils and the Angiris Council. It's a really good book, I can't wait til it's available.
Not so much full color artwork as concept art. It's meant to be written by Cain, so it has notes, drawings, and things like that. If you want full color artwork, the Collector's Edition of Diablo 3 comes with an art book that might be more your liking.
At the San Diego Comic Con, Chris Metzen described this book to be as sort of an encyclopedia of everything Diablo universe: Sanctuary, Burning Hells and High Heavens' lore, monsters, NPC, and locations. For those seeking pure full artwork -- as said earlier the Collector's Edition comes usually with a book of full color concept art. A year later or so, the book is available as stand alone.
Here's a transcript of Chris Metzen at SDCC 2011 talking about Diablo III: Book of Cain: http://www.blizzplanet.com/blog/comments/sdcc-2011-blizzard-world-of-warcraft-starcraft-and-diablo-swag-show-panel-round-up
Chris Metzen also discussed Diablo III: Book of Cain at BlizzCon during the Diablo III Lore Panel -- full transcript here: http://www.blizzplanet.com/blog/comments/blizzcon-2011-diablo-iii-lore-panel-transcript