The Book of Tyrael, a work of Diablo 3 merchandise that uses a great deal of original artwork, plus story and lore about the world of Sanctuary to set up and flesh out events between the end of D3V and the start of D3X. It’s a sort of sequel to the Book of Cain, and it’s available now (or soon™), from online sources and probably in brick and mortal bookstores near you.
The official product description:
A few fans have reported copies in their hands already, and since I’m not Vaeflare I’m saying spoil away. Put full details about anything and everything interesting in comments of this post, and if you don’t want to read them, don’t click through.
If you want to catch up without risk of spoilers, check out the DiabloWiki.net Book of Tyrael article. It’s got more product details and a selection of sample pages from the book.
Update: HolyKnight picked up a copy at his local Barnes and Noble, says the book is gorgeous and took some pics that you can see in the Merchandise gallery. The big one is a fold-out, map-sized Leoric family tree, suitable for wall decoration.