@Galahad said
"For some reason, I was prompted to study the Yi King this morning and cast coins for the first time. I worked off the instructions and hexagram interpretations in Liber 216. However, I was unclear about what role the 'moving line' should play in interpreting a hexagram. For example, I cast a 39 today (Kien - Moon of Earth), with the moving line in Line 5 (second from the top) - what are some possible ways to interpret this positioning?"
My understanding and experience have revealed that the moving lines are a secondary step in the process. They may be able to indicate a more specific reference to the current situation that you are asking about. In this case, the moving line at 5 indicates that although you are struggling with an obstacle(s), friends will help.
Once you've analyzed the question and response thoroughly then you can look at the hexagram that the changing lines create. In this case the solid line in the fifth position changes to a broken line, creating Hexagram number 15: Chi'en/Modesty. This gives you an idea of the future situation if you heed the advice from the initial hexagram. The I Ching primarily tells you what's going on now and what* could* happen in the future, just like any proper oracle.
@Galahad said
"In Liber 216, Crowley also refers readers to another book, "Magick," where he apparently discusses the Yi King in greater depth - does this refer to the brief discussion of divination in Magick in Theory and Practice? Or is there another source I should look for? Other, non-Crowley sources would be appreciated, as well. "
Although I appreciate Crowley's interpretation and utilize his perspective in readings, I find it extremely valuable to reference multiple sources: Wilhelm, Blofeld, Legge, Karcher, Wing, et al... as we are translating not only a language, but an entire abstract symbol set from an ancient and distinctive culture. You can find pdf's online but I also suggest getting yourself a hard copy.
I hope this helps and please feel free to ask any more questions about the reading. I absolutely adore the I Ching-It's never steered me wrong!