The Kybalion is the core around which the Master Key System builds and expands. Officially it's authored by the Three Initiates, but chances are that it's been written by one person only, namely William Walker Atkinson.DOWNLOADYOUTUBE VIDEOS
The Kybalion is way more abstract than the Master Key System, and will be enjoyed most with some esoteric and spiritual background information. It extols on the 7 Hermetic Principles, namely:
6. Cause & Effect
These 7 principles are the absolute foundation and cornerstone of our entire existence, and knowing how to "deal" with them greatly improves upon your quality of life.