Short and sweet and to the point. I've read other books that went into more depth, and covered more ground, but Nancy Barnes' efforts are worthwhile. Because of her more focused topic, it's easier to keep straight the lessons being taught. Contained quite a few concrete examples, some of which I bookmarked on my Kindle to use as reference material in the future. About the last quarter of the book covers a few other areas people have trouble with, such as affect/effect, you and I/you and me, and lay/lie. Obviously, this section strays from the primary function of the book, but it is tucked into the back of the book in a quick reference format and does not distract from the main topic.
So, while there isn't anything new here, it's presented in an accessible manner, and concise enough, it's easy to use as a reference. I'll be keeping it loaded on my Kindle for those all-to-frequent times when my brain refuses to function and I need a quick refresher on avoiding passive voice.