Well, you don't need a receipe book to create a new meal. Sure you read up or learn a few recepies, but then you can experiment and try out new things and combinations. You can apply the same to magic. Although I agree that a skill tree would be more appropriate.
In practical magics, while you can purchase spell books that detail the ingredients and rituals you need to cast things, there are also those books that describe what the base elements mean. By using these base elements, you can design your own spells tailored to your needs. Also, magic requires a lot of personal energy (like mana). By designing your own spell and putting all of that effort into it, it consequently puts more of "you" into it, incresing it's effect.
