This is the second book I've read by Blocher, the first being "Evil and the Cross." Both books are short, but not easy reads. Perhaps this is because the topics are difficult. Perhaps this is because Blocher is a Frenchman (writing, in this case, in English, while the former book was translated from the French). Perhaps it's because Blocher isn't a very clear writer. Perhaps this is because Blocher is far more learned than I am. I suppose it may be a combination of all these things.
In this book, Blocher attempts to "illuminate the riddle" of original sin. (The subtitle is, indeed, "Illuminating the Riddle.")Blocher does not claim to answer all questions regarding original sin. Mysteries remain. But he does try to show how we can inherit a sinful nature from Adam though not be imputed with an "alien guilt." It seems to me that Blocher's conclusion is that because of Adam's (and Eve's) sin, humankind was deprived of fellowship with God. When Adam and Eve sinned, they had to leave the garden of Eden. Thus, all the rest of humanity was born "east of Eden," outside the direct presence of God. No human (save Jesus) is born with a relationship with God. This lack of fellowship results in a sinful bent in all of us; from the earliest stages, we are bent towards sin (though, still reflecting the image of God, we're not as bad as we could be). I say "it seems to me," because Blocher never makes this as clear as he should. It would be helpful if we re-articulated his findings in clear language.
The strengths of the book are interactions with historical theologians and Blocher's analysis of Romans 5:12ff. The weakness is the lack of clarity. It seems that Blocher often assumes the reader knows what he is talking about.