Building a magic system on threat of destruction

Jack is a simple man. He reads for fun, which means he isn't picky. This fact probably saved his life when some kind of god type figure grabbed his soul and demanded help. Forced into servitude for a god that would like to be rid of him as soon as possible, Jack has to figure out how to survive. His solution? Building a system for an entire world of people, and hopefully not accidentally breaking the entire world in the process.