Memorization isn't super important, as it'll just multiply the amount of stock you get by 100 + your skill level. This is probably the difference between getting 70 stocks and 73 stocks at low levels. Basically, you won't be seeing anything noticeable until probably 50 (that'll mean 1.5x). Spellbooks are much cheaper in E+
Meditation will just rise on its own as long as your MP bar isn't full, so it's also a non-issue. The real problem is keeping its potential high so that it actually gets more than a tiny scrap of exp a turn.
Casting is somewhat important as it makes it more likely you'll succeed at actually casting, but it's not everything. Leveling the spells individually will give you 1% better cast chance per level and cast chance caps out at 90% if you don't level your spells, so you don't really have to go out of your way to raise Casting as long as you pump the spells instead. It doesn't raise damage for us players, but it does for NPCs.
Literacy is probably on the more important end of the scale. No literacy means no higher-end spells: no domination, no speed, and no cure of Jure/Jua/engrish. You'll definitely want to get up to at least the ability to read books of speed, because speed is awesome.
Control Magic is a pain in the butt, but very important. Personally, I just blew it off, but that's a terrible thing to do. You'll probably want to raise it concurrently with your bolts and ball spells, because someone on the wikia said you need more CM for spells to pass through your allies the higher level the spells are. If you wait until your spells are super-strong, they won't pass through your pets and you'll probably just kill them. From what I saw, it certainly looked this way. The more pets you hit with balls/bolts, the faster it'll train, so it's best to get all your pets out and then spam balls on them until they almost die. Be sure to give them resistance gear so that you don't kill them instantly.
Magic Capacity is pretty easy to raise if you spam a lot of curing spells into the negatives. It'll get even easier if you get your hands on the Magic Canceler god reward thing from Itzpalt. guessingo wrote about this a few pages ago.
What I do is learn the spells and then never use them until I've slept enough to get all the ones I want to use to 400% potential. After that, I just spam each spell until it levels once dropping to 360% potential. It goes pretty quickly if you just hotkey them and hold the button down. If you want, you can stagger them out by types (hexes, buffs, attacks, heals) so that you don't get periods where all your spells are at 380% and it'd be a waste to train them compared to 400%.
You definitely want some spell enhancing stuff. At least get the Staff of Insanity. That'll boost damage, hex and buff durations by about 50%.