I must first declare an interest. I know and like Bill Darlison, and we even share a birthday, which could be significant given the theme of the book. On the other hand, I am an extreme sceptic, particularly about astrology. There is evidence, at least for northern hemisphere temperate zones, that the season of your birth can have an influence on your psychological profile, which sort of makes sense and which would explain most of the apparent successes of horoscope casting. But as for the stuff about "constellation X being governed by planet Y", I have just two words to say, and one of them is "jumbo".
But in a way this is irrelevant to Bill Darlison's main thesis, which is that Mark's gospel is packed with zodiacal references, albeit coded so that only the cognoscenti of the time would pick them up. What is important is that the wise men of the first century certainly did believe in astrology.
The book's presentation of the zodiacal structure of the gospel is pretty convincing, so we are forced to conclude that one of the (many) readings of Mark is as a sort of month-by-month "self help" manual for spiritual development. As we pass through the year we should learn to use the strengths associated with each sign, while trying to avoid the weaknesses. But of course this is good advice even if you don't believe in astrology. Or even if you wouldn't call yourself a Christian. Read it whoever you are!