Spell tomes should be able to be used when you have the spell.
Like the schematics you find in fallout. If you find more the better the made item gets.
Even if they just iincreased damage by10% and you could only do that 1 or 2 times I'd be happy.

When you read a recipe to make tortillas and you read it again, will you be able to make even better tortillas? No. That's how this works. They teach you the spell, but that doesn't mean you get better at it when you read the same basic instructions about how to cast it again.