SE version can be found here.

FEATURES:

• New power: Spell Steal, which lets you permanently learn NPC spells
• Learn up to 50 NPC spells at a time
• Individual spells stored by this mod can be removed at any time in the MCM
• Stolen spells can be equipped with either hand or with both hands without changing the AI's behavior
• Stolen spells are tagged with the appropriate casting perk
• Assign magic schools to stolen spells that aren't already associated with a magic school
  

CUSTOMIZATION:

• Skill requirements: when enabled, you can only steal spells that are within your skill level, e.g. you need a destruction skill of at least 25 to steal Firebolt
• Level-based spell slots: when enabled, you can store up to X spells at a time, where X is the player's level (or 50, whichever is smaller)
• Make Spell Steal a Lesser Power: by default, Spell Steal can only be used once a day. Enabling the Lesser Power option allows you to cast it multiple times per day at the cost of magicka
• Show/hide the magic school assignment dialog box when stealing a spell that doesn't have an associated magic school
  

WHERE IS THE SPELL TOME?

The spell tome may be found randomly with low probability in warlock boss chests once your character reaches level 8 or higher. There's also a guaranteed copy in

BUT I HEARD NPC SPELLS CAN BE WEIRD! I DON'T WANT WEIRD SPELLS!

That is true, but this mod technically adds a copy of the NPC spell that is modified as to eliminate some of the strange properties you usually see in NPC spells. For example, many NPC spells can only be equipped with the left hand (or right hand). The script that runs when you add a new spell automatically checks if it's a single-handed spell and, if it is, sets it so that it may be cast in either hand. Since this is all done on a copy rather than the original, these changes only affect the player and will not interfere with the AI (those spells were designed to be single-handed for a reason, after all!). Likewise, if the spell doesn't have an associated magic school, you'll be prompted to assign one and the appropriate casting perk will be assigned. And if you ever feel your spell book is becoming cluttered, you can delete some of the stolen spells via the MCM menu!

Additional notes on how this mod handles degenerate NPC spells:

• Fire and Forget spells with zero cast time are given a cast time of 0.5 seconds
• Spells with no magic effects are not stolen
• Constant effect spells are not stolen; only Fire and Forget and Concentration spells are stolen
• Spells with zero magicka cost are not stolen at this time. I may add this functionality in a future release if I come up with a way to intelligently assign spell costs for zero base magicka spells
  

CAN MODDED SPELLS BE STOLEN?
Absolutely! No patches are required for this, assuming the spells are equipped by an NPC and have at least one magic effect and a nonzero magicka cost.

KNOWN ISSUES:

• You may end up with spells with duplicate names or that appear identical. The mod differentiates spells based on Form ID, so e.g. if there are 3 spells called Firebolt with the same magnitude, cost, etc. but different Form IDs, they will be considered distinct and you can steal all three of them. Because the spells are copied and modified they may seem indistinct to the player, but in reality one might be the right-hand only variation and another might be the left-hand only variation. Work around this by using the MCM to remove unwanted duplicate spells.
• If you steal a concentration spell that isn't associated to a magic school and assign it as a Lesser Power by choosing "No School" in the dialog menu, the spell will interrupt almost immediately when you try to cast it due to the Lesser Power cooldown. Work around this by assigning a magic school.
  

REQUIREMENTS:

• SKSE 1.7.3 and Skyrim runtime 1.9.32 - This will never be optional as this mod uses a custom SKSE plugin
• SkyUI
  

COMPATIBILITY:

• This mod changes one of the leveled lists, so make a Bashed Patch or you may run into incompatibilities with other mods
• Exclude this mod from the ASIS patcher if you're using ASIS
  

RECOMMENDED MODS:

• My other mod, Your Soul Is Mine - Soul Gem Relinquishment (Newly available on Legendary Edition!) if you want the full Shang Tsung experience! Fun fact: this mod was originally planned as a feature of that mod, but was cut before release to maintain minimalism
• A mod that adds more spells to NPCs, e.g. ASIS
• Anything by EnaiSiaion
  

CREDITS:

• The SKSE team
• Ryan McKenzie, whose SKSE plugin writing guide and examples I used to make the plugin.