I was just doing a run through on an Oblivion gate, and as I was creeping up on an enemy, I cast a regular Shield spell (Defend - Shield 15% on self). When I engaged the enemy, I realized it was a Flame Atronach, so I quickly cast my Heat Shell spell as well. I noticed that both effects were in the corner of my screen, and when I checked my armor rating, it said 40, when my regular armor rating is 16.

Could I have missed, all this time, that Shield spells (of different effects, of course) stack with each other?

Yep, they stack. The only thing that won't stack is casting the very same Shield spell over and over on yourself, as this will only reset the timer. But, you could create 2 Shield spells with the same effect but a different name, and those *would* stack with each other.

Shield enchantments on equipment stack with each other, as well as with any spells cast. You can run around with an AC of 75 and 25% resistance to all 3 elements just by enchanting 3 pieces of clothing with elemental shield sigil stones. Cast any Shield spell on top of those that confers at least 10% more shielding, and you'd be at the max.

Wow, for all the years I've been playing this game, and I never realized that. I always thought you could only have 1 Shield spell at a time. I love the Shield line of spells even better now, lol. With this discovery I've more than doubled my AR. It's totally changed my gameplay.

You into Alchemy? Won′t say too much but there′s a lot to gain from that

So 25% Fire resistance yields 25% shield and 25% fire resistance on top?

No. Fire resistance is just fire resistance. Shield is just shield.

The effect Fire Shield provides both at the same time. More info here: http://www.uesp.net/wiki/Oblivion:Fire_Shield

There is also a Frost Shield and Shock Shield effect that work similarly. These three effects are sometimes collectively referred to a elemental shields.

Didn't know most of this. Sweet.