Once you get fireball there's no reason at all to use fire bolt unless you're fighting really easy shit and don't want to waste your mana. Even single target I use fireball over fire bolt.

I've got Holy Bolt at level 14, and 5 books of Holy Bolt sitting in town because they take 255 magic to read. Once I reach that point, I'm going to read them and see what happens. I can't remember if it stays at 14, goes up to 15 and maxes out, or if it keeps going up to ~20 or whatever.