This was the first book in English to discuss the Tai Xuan Jing and its 81 tetragrams. So it deserves credit for that.
However, there is a careless error in the chart of tetragrams which I have seen perpetuated by other Internet writers using this book uncritically and reproducing the faulty chart from the book.
The chart in the book shows the unbroken line for "Heaven," the once-broken line for "Man," and the twice-broken line for "Earth."
That is wrong. The correct sequence is Heaven, Earth, and Man. It is "Man" that has the twice-broken line. "Man" is third, after "Heaven" and "Earth", since "Man" is the offspring.
The Chinese text, and the later, scholarly translation of the Tai Xuan Ching (by Michael Nylan), has the correct correlations.
This is important in studying the tetragrams correctly. Otherwise you can write an entire essay and be off-the-mark with your conclusions. So, just to review, Derek Walters (or the publisher) made an error in his chart, and the correct way is:
solid line for "Heaven"
once-broken line for "Earth"
twice-broken line for "Man"