Having just embarked on a fairly intensive writing course, I asked my mentor for some recommendations of books on the craft of writing. This book, "The Sense of Style" was at the top of his list, and I can see why.
The author, Steven Pinker, is a Psychology professor at Harvard, and has also done much research on language and cognition (he's described as a Cognitive Scientist). Further, he is chair of the Usage Panel of The American Heritage Dictionary. And it shows. All of it. He has written a very readable, engaging, and entertaining manual on writing, providing a wealth of information which will help anyone write better prose. He also goes into considerable detail on the mechanics of good writing, including syntax, noun piles, quantifiers, remote conditionals .... you get the idea.
Even though he goes quite deep on the technical side, he is very generous with clear, helpful examples of everything, a good many of which are laugh out loud funny. I found the book to be a solid 4 or 4.5, but the final chapter, "Telling Right From Wrong", was a lengthy and very practical compendium of many additional illustrations and applications that took it to a 5. If you are interested in becoming a better writer, you will benefit from this book!