Skillbooks on vendors are random. If you haven't found one yet...well that's simply a case of bad luck. While summons do soak up aggro, they're not strictly necessary. You can get along fine without them. Me and my friend are going double lone wolf on Hard with no summons and are progressing just fine.

As for enchanting, enchanting weapons with any elemental damage requires crafting 5, but basically goes like weapon + fire/water/earth/air essence -> weapon w/ bonus elemental damage. You can also add poison damage to a weapon, which only requires crafting 1: weapon + poison.

My super-generalized combat tip for you is that CC and damage mitigation are king. They are much more important than DPS and work a lot better unless you are going for very specific cheese builds like glass cannon xbow sniper.

Also use environmental effects to your advantage. Enemies generally will avoid fire and poison surfaces like the plague (unless they are immune, or already burning/poisoned); use them to create choke points or split a large group into two. Smokescreen (and smoke in general - create it with poison + fire or oil + fire) is also amazingly useful for controlling aggro (enemies won't bother moving to get vision on someone in/behind smoke if they can see someone outside of smoke) and for creating chokes (enemies will move forward into smoke if they can't see you).

And a final note: all the shield spells (Fire Shield, Earth Shield, etc) are horrendously overpowered, so use them if you're having trouble. The tooltip doesn't tell you that the shield blocks ALL damage until it has taken a certain amount of damage or times out. It's basically a massive HP boost with 100% uptime if your mages have any kind of decent intelligence stat. And the shield's health is restored to full upon re-application, turning the spell into a massive heal as well.