Casting is a magic based skill that reduces the chance of casting failure when casting spells. This skill can only get spells to a maximum of 90% success. The last 10% has to come from leveling the spell. Each spell has a difficulty level associated with it, which influences how difficult it is to read the book in the first place, as well as cast the spell when you have stock. The more difficult the spell, the more it trains casting. Magic Dart may be cheap and common, but it won't give you much casting experience. Elona+ made it so NPCs can use it to power up their spells.