Joseph Williams' book, "Style: Toward Clarity and Grace" is the best book on writing I have ever read, by far. Williams himself describes the emphasis of the book on page one: "Telling me to 'Be clear' is like telling me to 'Hit the ball squarely.' I know that. What I don't know is how to do it." But Williams does know how to write well, and his explanations are precise and concrete.
This book takes a sort of linguistic, almost scientific approach to improving your writing style. I first learned of Williams' work in "The Language Instinct," by the Stephen Pinker, the acclaimed professor of linguistics from MIT.
Unlike every other writing book, this one is more than a laundry list of grammatical shoulds and shouldn'ts. This book is about HOW-- how to write to suit the human brain's innate method of processing information.
I am a professional writer, and I have a whole book case filled with grammar books. But this book is worth more than all the others combined. If you're a writer, this is the book you've been looking for.