I think rats and skeletons are most useful as cannon fodder, ie to provide a distraction while you use ranged attacks. I did not used summoned creatures at all as a warrior; if you are playing melee, I don't think rats/skeletons would be much help, from what I've read. You might want to consider Resurrect as well (I maxed it with bought spell books), if you want someone following you around (summoned and resurrected creatures both require Aura of Command for that).
You can buy mage spell books. Good merchants to check are Kistandalius or Blake in Ars Magicana and Corinna or Mpenzak in Verdistis. If you quick save before talking to a merchant, hitting alt or running your mouse over them, you can quick load if they do not have what you want.
