This, SWH, and Loves Me are, I think the three most influential books I've read since my time at L'Abri. I really appreciate this book; Plantinga is an engaging writer and does well covering the essentials of a difficult subject.
He gives a helpful working definition of sin and makes valuable distinctions between different categories of sin; he talks about the relationship (and the difference) between sin and folly and handles the tricky 'sin for me' (but not necessarily for you) aspect of sin. You'd think a book all about sin would be heavy and depressing, but Plantinga is positive; honest, not naive, but gracious.
Something I really benefited from while reading Not the Way was the way reading about sin helped me refocus on my sin. And Plantinga mentions in his introduction that this is one of his objectives: to reintroduce the subject once again for serious contemplation. He's right; partially (as usual) in reaction against the extremism exhibited by the generations before us, and partially because it's much more comfortable, the generations of the Baby Boomers and beyond focus on the Image-bearing goodness of human kind at the expense of the sin-bearing seed we also carry. We look the other way or gloss it over; we joke and shrug it off. It doesn't grieve or anger us. Dessert is sinful; I just make mistakes.
As you can tell, I recommend this book. Pick it up.