Dear Jack,
Thanks for reading my books. The five elements come from the interaction of Yin and Yang. Yin and Yang are the dual forces of nature. It can be reflected on all aspects of life in our universe. Through this interaction, 5 Phases of energy patterns are formed. These are called the 5 Elements.
The 5 Elements in our solar system refers to the 5 major planets - Mars, Venus, Saturn, Jupiter and Mercury. Their movements in our solar system together with our planet, influences the flow of Qi.
There is a classical book all about the 5 Elements called the Wu Xing Da Yi. You can try to obtain a copy in specialty bookstores selling metaphysic books.
Hope this helps :-)
Warm Regards, JY