As the progress bar on Reveal.diablo3.com reaches 20%, a new unlockable is revealed. This time it's an excerpt from the "Book of Cain" on the Archangel Tyrael.
You can read through the page more clearly here and after you do that, maybe go and like photos and art on the site to push the progress bar further. As of 19:00 CET on April 13th, the site is at 24% so six more percent till the third secret.
Maybe another part of the Developer Diaries is next...
Unlockable #1: Developer Diaries Part 1
