Apologies for asking something that is probably obvious to most of you, but I just got Oblivion and I can't seem to find an answer to this one by normal searching/Googling.

If I'm wearing a robe that gives "Shield 18%" but offers no armor rating of its own does that mean I get...

a. The rest of my armor pieces boosted by 18% (which isn't much, as I've just got some weak gloves, shoes, helmet, and shield, and not highly leveled armor skills)? Or...

b. 18% subtracted from incoming physical damage?

Thanks in advance.

I believe you recieve 18% protection from incoming mele damage

It is in fact 18% Protection on any incoming physical damage. I normally use an enchanted Pendant that has a 25% Shield on it aswell as using a custom Barrier spell to give me an extra 60% Shield. It's very nice and is worth using items and spells like those if you are new to the game. Some people say it takes away from the game to use over the top shields but I can still get splated quite quick by things if I'm not careful.

Yes, it would be 18% protection from damage. You can have a maximum of 85% protection.

I'd point out that at 18%, that one item would offer more protection than a full set of fur armor, and if your armor skill (and the condition of the armor) aren't very high, it might well be better than a set of low-level heavy armor.

Now what if I already have 85% protection with armor is additional magic spells like Agies (spelling) gonna protect me further?

Magic can be 100% resisted, absorbed or reflected.

But you can't get more than 85% resistance to physical damage, however you stack it. You can get 100% reflect, and there's no "absorb" equivalent.

Awesome, thanks!

Yes and no. It's not going to be of any benefit at the start of the battle, but as the fight goes on your armor will get damaged and that will reduce the AR it gives you. At that point the extra protection from Shield spells and enchantments will help you.

I've created a custom spell for mastery in Alteration with includes: 50% Shock Shield, 50% Fire Shield , 50% Armor Shield. Does the 85% limit stop the magical effects also?

You will have a limit of 85 for melee damage. But you will still get a full 50% reduction in Shock and Fire attacks.