@Dark Brotherhood: After you join them, look on the shelf behind the leader. There is a set of Mage DB gear there.
@Illusion Magic: Illusion took me forever to figure out, but it was on a thief who wanted to expand his skillset, It's a support magic that is best used in conjunction with sneak skills. If you plan on using it at all, you have to pick up Dual casting IMMEDIATELY though as it will double the max level that illusions work on.
Frenzy: works up to level 6 normally, up to level 12 dual casted. The trick on this one is to use it from the shadows. Imagine two bandits, you hit one with frenzy and he will attempt to kill his buddy. One of them will die and the other one will be heavily wounded. All the while they are not actually detecting you, which adds to your Sneak skill.
Calm: Prevents an enemy from attacking you, allowing you to move in easily for the backstab. This can also prevent an enemy from pursuing you.
Fear: Frenzy and Calm are good to use in stealth to control the situation, however Fear is best used when things go sideways. A well timed fear spell can either turn a "2 on 1" into a "1 on 1", or it can buy you time to heal up if you take too much damage. If you use Destruction or Archery you can range them to death as they run away too. Doing this with melee is a bit of a pain, but works better if you use a "Slow" poison or "Slow Time Shout". This is commonly called "Fear Kiting" in other games.
The big trick I learned is that Illusion almost needs to be dual cast to land the effect (max level x2) as you will likely outlevel the single cast effect long before the perks become available.
One of my upcoming characters is planning on being Bow/Sneak/Illusion/Alteration. I'm not sure if the next one will be that or Heavy Armor/Destruction magic. I try to keep my designs so that I use different "guardian stones" for offense and defense, allowing me let one rank up faster if it falls behind. Nothing worse than realizing that your defense didnt keep pace with your offense.