The main text of this book "Secret of Gold Flower" deals with both an age old meditative practice and a supportive theory in the Taoist tradition. Carl Jung's commentary basically dealt only with theory part and interpreted as similar in concepts to his individuation process (the development of the self as similar to development of the golden flower). It adds credibility to both sides: Taoist practice gained western academic respectability whereas Jung's concepts of collective unconscious and archetype gained supportive evidence from an old high-culture. It is interesting to note that whereas Wilhelm was a Taoist practitioner who had brought himself full-scale into Taoist yoga, Jung on the other hand advised against western people taking up such practice!
On the practice side, the choice of texts is excellent: "Secret of Golder Flower" deals with the leading role of "human nature" (collective unconscious)in Taoist meditation with full supportive theory, whereas "Hui Ming Jing" deals with the procedural details of the practice itself (a combination of Taoist and Buddhist practice, the author himself a Buddhist monk of Zen tradition). Though an English translation might not be able to present and differentiate finer points that can only be understood if one can read the Chinese language, one benefit is that a translation can do away with some unnecessary mystical elements (for example in the book symbols from I-Ching are replaced by alphabets)that often times leads would-be practitioners into unnecessary maze. Though I have to complain that more of the Hui Ming Jing had not been translated (included) for a more complete manual for practice reference.
For those who are interested in the texts as a living practice, do not miss Wilhelm's excellent summary on the practice of Hui Ming Jing quoted by his wife in her Forward. For those who seek for historical similarity between this practice and similar western practice (now for all intend and purposes is extinct), do not miss Wilhelm's "Discussion of the Text" concerning past persecution in China towards some serious practitioners due to their (sometimes unavoidable - guess why!) linkage with political intrigues and peasant uprisings. In comparison, similar persecutions in the West had been more religious in nature.
On the psychological commentary, Jung is as usual, profound in his analysis. More of his analysis towards other Eastern text can be found in his Psychology of Eastern Religion (a collections of his articles on the subject matter - also highly recommended).