All we really know about blocking right now is that 1) you move slower 2) it costs stamina to hold down the block button, like sneaking 3) you can block projectiles/ranged attacks as well as melee attacks. I'm not sure if you can block spells/abilities, but I don't think we'll be able to.. 4) you can block with weapons other than shields, but maybe not dual-weild..although I reckon we'll be able to. 5) you can bash, it costs stamina, and you can also power bash with the ability in the shield tree 6) I don't believe there is parrying 7) when you block someone's power attack it puts them off-balance, and then when they get hit by a power attack they get knocked down.
I don't know how accurate this information is. "Projectiles and ranged attacks" might not include magic abilities, but rather abilities like Scatter Shot, Silver Bolts, or Hidden Blade as well as arrows.
I'm not sure the base % that block reduces on damage, it's probably something like 50% for both projectiles and melee--because there are a lot of abilities that increase block % either as a whole or for specifically melee or ranged by quite a lot.
I don't think anyone has this information without violating the NDA. So I'd recommend asking for a PM.