The easiest way to kill him is to use an area effect spell that does not need a target, such as Poison Cloud. If you cast it near, but not directly on him, it will do damage without turning him hostile (so he will not counter attack) or giving you a reputation drop (usually -2 points for attacking a friendly NPC, IIRC).
Spell books can be found in bookshelves randomly, or you can wait until you level up high enough (level 20), or reach Ars Magicana to try to buy a spell book. Seth disappears once the escort quest is complete, though.
