Contingency:

Helpless? Contingency can save you with a defensive spell, then when you are level 18, you can use Mislead instead. Contingency allows you to store one spell for emergency purposes. At level 12, you can store up till a level 4 spell, at level 15, up till a level 5 spell, and at level 18, up till a level 6 spell. You can also use it as a battle opener, storing spells like Protection from Magical Weapons set to trigger when enemy is sighted or when your Sorcerer is hit.

Death Spell:

Sick of fighting hordes of Minotaurs and Umber Hulks? Couldn't be bothered to waste an Abi-Dalzim's Horrid Wilting on another patch of Gibberlings or Orcs? Cast this spell and all of them will drop dead immediately.

Protection from Magical Energy:

Wish to protect your party members from Magic Missile, Skull Trap and Abi-Dalzim's Horrid Wilting? Then this spell will help a lot.

Mislead:

This is easily one of the most easily abused spells ever. Cast Mislead out of the enemies' line of sight and proceed to whack them for free. Even if they cast True Sight, the invisibility will not be dispelled because they must have the illusion in their line of sight to dispel it.

Chain Lightning:

This spell is good for hitting multiple enemies as it is an area effect spell. Pack 3 of these in a Spell Trigger for more fun!

Invisible Stalker:

Skeleton Warriors are far better summons.

Pierce Magic:

While potent in the beginning, it loses its' bite in high level fights. Sorcerers do not have the luxury of changing their spells, so it is better to wait for Ruby Ray of Reversal, Pierce Shield and Spellstrike.

Tensor's Transformation:

A Sorcerer's true power lies in his spells, not in his melee skills.

True Sight:

I chose to skip this spell because I wanted more firepower, while the others can cast it instead. This is a vital spell for soloing Sorcerers though.