Warning: considerable amounts of rambling ahead, along with some clean sections
Spells are inscribed onto scrolls and then cast, nothing fancier than that. The implementation of it is complete for the most part, with the remaining bits being adding a bunch of spells (in addition to the already existing 11 ones), making the scroll tier have an effect on the spell, and likely some tweaking the inscribing process for upgrading spells (all of the code is in place, it simply hasn't been tested due to the lack of spells to upgrade).

Inscribing the spells and casting them are both fully implemented. The spell section in the Necronomicon needs some touch-ups (maybe some pictures, but primarily more descriptive on how casting works, since that seems to still be unclear to some).

Spell glyphs need to be implemented, so that each spell has a unique glyph rather than each having the same (but with a different color).

Some usage of the knowledge system will also be added down the line, simply so that you have to do something in order to unlock the spells (apart from any spell that requires a specific Necronomicon, but none currently do).

Things to do

• make the scroll tier affect the spell to some extent
• rework the information about spells in the Necronomicon
• add more spells
• implement spell glyph rendering on the scrolls
• incorporate the knowledge system, requiring the player to unlock the spells

As with any issue tagged with development, any input and/or feedback is appreciated, or you could just bitch about it on Reddit (and I might come across it at some point in time).