INT headband (first major buy, always)
More spells for your spellbook (scrolls if necessary, but you should be able to buy access to spells to copy cheaper)
If you can burn even a couple days, write your own scrolls of situational spells you won't necessarily prepare.
You're right, pearls of power can be very useful.
Don't forget a couple alchemical power components (liquid ice adds +1 damage to your ray of frost cantrip, for example)