Also , do you get the guy's mask and robes?

Yes you get his robes and weapons and a little something extra

Perk Reset 10 Souls & his Staff

Armor that is enchanted with crappy enchantments.

I hope there is a blank version in the files, if not then... Why.... Just why??! Nightingale Armor take 2...

Before facing him, you'll get another word for your Dragon Aspect shout. After you defeat him, you can pick up his unique armor, staff, and sword. You also absorb his dragon souls.

His sword does damage to stamina. Not too great, but it looks cool.

Miraak's staff sprays that black stuff on the ground, which has tentacles coming out of it.

His mask, gauntlets, and boots are light armor. Unfortunately, you can't improve their armor rating. The robes have no armor. When you wear them all, there's a chance of spawning a black tentacle to attack your enemy. All pieces reduce damage from shouts and magic, which I imagine must be useful against dragons.

The Black Book lets you choose to clear perks from whatever trees you choose.