
Bashiok was kind enough to warn those eager to receive their collector's edition and devour its content with hungry eyes: Stay away from the art book and the behind-the-scenes DVD until you beat the game on normal if you want to avoid getting spoiled.
You should be especially cautious on May 14, as with the installer unlocking and worldwide releases taking place, the entire game will be available to be discovered and datamined. We also plan to populate the Diablo3.com game guide with item data that Monday, which may themselves be spoilers. If you care about spoilers, it may just be best to not go on the internet on May 14.
This is nothing new, of course, and the same was true for StarCraft 2 as well. But you know, it never hurts to be cautious!
Source: Battle.net