英語はある程度できる（書ける）けれど、native checkがうけられないのでクオリティが心配、という方にお薦めのハンドブックです。
たとえば文章のtoneなどについて、すなわち辞書で調べてわかるというものではない感覚的な要素についてのガイダンスがあるところが一番役立つ点だと思いました。
説明は全体に簡潔で読み易く、また事例も載っているのでいわんとしていることのイメージがつかみ易いです。

Although I am not in a business-oriented profession, this book offered me quite a bit of knowledge in professional writing. I am a technical writer so I do not follow the business format when writing reports, but the rest of my daily writings greatly benefited from this book.

By reading this book, you will soon understand why the writer is truely qualified to do so... Many people have to write on the job but are you doing it well? Is your message getting across? There must be a better way to do it. This book is definitely worth the cost to the business writer. It's cheap and very well written in an easy to follow style. Written in 1996 and now in the second edition, this book is still modern enough to cover the all important topics of E-mail letters and new technology. If you're unsure of whether you are writing well, or you just want to brush up your business writing, this book is a good place to start.