Are they written in some sort of magical language that somehow transfers the basic knowledge of how the spell is supposed to 'work' when you read it? How do people write spell scrolls/tomes, and what type of people would write them? Realistically, spell tomes wouldn't disappear when you 'use' them (IIRC, in Oblivion, they didn't disappear like they did in Skyrim), but would spell scrolls burn up or disintegrate upon using them as they do in-game?

Scrolls are single use enchanted items. They most likely get destroyed when their powers are released. It's just paper after all.
For spell tomes I'd say gameplay decision. So you can't sell them after reading.