I am giving this book a middle rating, as whether you think it deserves to be higher or lower will depend on the audience with whom you plan to use the information it contains.
You will want to raise the rating if you are communicating with the typical, Western, twenty-something, semi-postmodern, who has absorbed a simple understanding of karma from sources in popular culture. They will not be aware of the corners that Herringshaw is cutting and will not be phased by the sometimes-adolescent wordings or odd Biblical/theological exegesis in a few instances. They should have no problem following the author through to the real point: that a Personal Being, not an impersonal process, is behind the world and is willing to come to our rescue. If this is your situation, you will find a lot of useful ideas and ways of saying things that will help you connect with such people.
You will want to lower the rating if you are communicating with someone who prefers a more mature level of language or who has an educated understanding of karma, particularly if they are a well informed Hindu or Buddhist. They will notice the incomplete understanding of one of their key beliefs. In fact, if they have an apologetic bent, they could easily take the sin/karma equation in this book and, like they can do with the Jesus/bodhisattva line being picked up by some Christians, turn it around against Christian beliefs in ways you never saw coming. This happens simply because most Christians do not understand these Eastern religions well enough to predict where the logical outcome of these miss-steps lead from the perspective of these worldviews.
Regardless, the second group is not the one at whom this book is aimed, though from the title and descriptions you might think it has a broader target audience. My comments are to inform the potential reader about its real market and help prevent disappointment. I am not trying to criticize the author for not doing something he does not appear to have set out to do, though on a few pages he does raise points about Hinduism and Buddhism. Regarding his comments on the latter, where I can speak with some authority, they do not represent how an informed Buddhist would see it.
You may also wish to adjust the rating depending on your tolerance for stories and illustrations. If you find such things mostly filler and like an author who gets to the point and moves on, lower the rating. The real content in this book could be made in much less space. However, if you love it when narratives illustrate each step, then this won't bother you. In fact, there are some very touching stories that poignantly illustrate what Herringshaw is getting at.