The book is called 'classic' and at least two meanings of the word apply, when describing it.
First of all, it is reasonably comprehensive and will cover the needs of people writing everything from a half page memo to a book - if you limit yourself to one book on writing, this may very well cover all the aspects necessary.
On top, it is classic in the sense that it reflects an earlier time - even this - the 50th anniversary edition was published back in 1998. This means you will get some quaint advice on the benefits of using a word processor instead of writing by hand, as well as how some card sort and early library computer searches operated, with the intention of making it easier to find the research material needed.
In spite of this, you will do yourself and the book a significant disservice if these odd passages persuade you to look elsewhere. The book is really excellent in its coverage, offers some well written, compelingly presented advice, has plenty of examples and a range of exercises in each chapter, which should help you along much more than barely reading the principles ever could.
It covers topics ranging from literature research, to planning, to structure, sequencing, to the use of paragraphs, connectors, correct use of specific wording, etc. As mentioned by the other reviewer, if writing is not something you are comfortable with (and even if you feel you are), the book will make a much more competent writer out of you.
It may not be typical management material, given that it is not always the most concise and that it covers a wide range of needs (far beyond memos, presentation scripts, etc.) so if this is the area of particular interest, something like Barbara Minto's
The Pyramid Principle:Logic in Writing and Thinking may hold more of an immediate appeal. Still, it will form a very useful complement to the much narrower focus of Minto's book.
What the book does not do, is form a guide to, or give tips and tricks on what you need to become a professional (fiction) author, which some readers may find implied in its title. It will give you more than enough skills to start on the path, though.