[Scottg]

You pretty much can NOT wear armor until Automatic Still Spell 3. You cannot put on armor during battle (or take it off).

There are however a few armors in-game with low spell failure penalties, still - it isn't worth it.

Key spells are:

Mage Armor, Shield
Mirror Image
Displacement, Improved Mage Armor, SpiderSkin.

You should almost always scribe these spells to scroll and then use the scroll to cast them. This gives you more attack spells to fill in your spell slots. This is particularly true for the "Armors", that have a long duration. Latter when you have access to persistent spells, you can use shield and Spiderskin as persistent spells if you wish.

Oddly enough the best spell of the group is actually Mirror Image, but it does work better with higher AC's and additional concealment (and is particularly sensitive to dexterity). Occasionally If I suspect a long tough battle I might use Mirror Image as an extended spell (you can't do persistent with it).

Note that in addition to your Dex. bonus you also can wear other AC enhancing items that won't cause spell failure.

Shields will also cause some degree of spell failure.. BUT when you start using them chances are you will be fighting, not casting spells - so its generally OK to just equip a shield when you need it (..something you can't do with armor during battle). In these instances just use the strongest shield you have, but remember to unequip it before casting another spell.

Finally you have the damage resistance spells:

Ghostly Visage
StoneSkin
Greater StoneSkin
Ethereal Visage
Stone Body
ShadowShield
Iron Body
Premonition
Shades

The StoneSkins and Premonition are somewhat like the "Armors" - they have a long duration and if possible should generally be scribed to scroll and casted from the scroll. Of course they only last as long as the damage they absorb, so sometimes you will also want *one* spell slot open for the strongest you can cast so that you can quickly cast it during battle in case your scribed scroll version was used up.

Ghostly Visage can be useful early in the game, but once opponents start getting magic weapons its pretty much useless. (..and you can get better concealment from Displacement and Greater Invisibility.. However it has a MUCH longer duration - so half the concealment bonus may well be worth it considering its duration length which is 10 times as long.)

Ethereal Visage for the most part is useless because its duration is so short.. BUT if you are about to go into battle that shouldn't take to long to finish then this is a good one to scribe to scroll and use just before the battle (if you can).

Shadow Shield is a really only good for Immunity to Death/Necromancy spells, the AC bonus can bettered from Improved Mage Armor. Furthermore with the higher fortitude you will gain with an Eldritch Knight class you might not need the Immunity to Death/Necromancy spells because you could well make your save.

Stone Body and Iron Body.. really need Auto Still Spell at their levels for these spells to be effective in most instances.

Shades is quite nice (mostly as a Premonition substitute).. BUT it requires either a costly scribed scroll or one of your precious 9th level spell slots.