I'm talking about one specific spell cast against you in battle. I.e., the opponent casts it, and it immediately gets written into your spell book. By levels, I'd make it: 2/3/4 levels of magic, depending on the skill level.
That would be quite a strategic skill.
And, as an option, you could combine Eagle Eye with Literacy: you could peek and teach another hero 2/3/4 levels of magic.
Attach this to Literacy, and remove Eagle Eye from the game entirely.
Added after 31 minutes
Another idea: make Protection from Elements a universal spell (i.e., protects from all elements) and tie it to Air. At level 3 of the Mage Guild, it would look nice. Again, this would boost Air, especially relevant for fighters, coupled with Haste.