So far in the World of Darkness, most forms of magic seem to be directly connected with being a supernatural entity (or finding a way to have dealings with one). Are there any rules that allow for Mortal spellcasting? Such as allowing them to use a ritual they found in a book. Maybe that ritual is voodoo, maybe it's ancient Latin, or Summarian. This effect wouldn't necessarily allow the to create their own spells, or rituals, simply attempt to cast them. Does anything like that exist?
There are a few ways to make that kind of thing happen. If you're looking for ways for mortals to be spellcasters that don't have a major template, that's in WoD: Second Sight. If you'd like there to be rituals that can be found in books, you might do well with magical items from WoD: Reliquary. Plus, there's a pretty good answer to this question about sleepers using scrolls and other imbued items.
They can use "Ritual Magic" p140 of Witch Finders
Magic is usually the result of the will of a single practitioner triumphing over the rules of reality to create spell effects. For individuals without the knowledge or ability to manage these effects alone there exists the option of cooperative magic, in the form of rituals. Ritual magic is a far slower and more tedious form of the art than that practiced by most witches, but it does work. In ritual magic, the will of the group (covens or cabals) allows the witches to cast spells that they wouldn’t be capable of individually. The practice of ritual magic gave birth to such concepts as spell components, day-long chanting and specific numbers of witches required to cast a spell.
It is important to note that anyone with a solid understanding of the occult (Occult Skill at two dots or more) can contribute to a ritual; possession of the Gnosis Merit isn’t required.