That doesn't mean I won't allow casting spells from a spellbook. It's just not like prepping and memorizing a spell.
What I would do is use the emergency spell prep time of 1d6 turns per spell, but double it, since the M-U isn't prepping then casting. This only works with spells the M-U knows and could prep. Even though the spell isn't being memorized, if the M-U doesn't have an "empty slot", there's a chance the spell is miscast: 5+ on 1d6, modify target # up or down for high/low Intelligence, but add the spell level to the roll. For the effect of a miscast spell, use the unmodified result:
- 1 to 4 -- No effect.
- 5 -- Fizzle. Minor ineffectual version of spell being cast.
- 6 -- Backfire. Roll for random spell of same level, apply to random target. If the random spell is the same as the original spell, the effect is reversed, if possible.