The title of this post is one of the most common questions I get from people looking to write their first book. It's usually code for, "I have to write how much?". Most people either have no idea how long a book should be (which is perfectly understandable), or they have a slightly off-centre idea.
Judging a book by its thickness
The thickness of a book is not really a good indication of how much work has gone into it. A book's thickness depends on a host of factors beyond the number of words. For instance, the paper used: the 'bulky cream' off-white paper often used by mainstream publishers is lighter but about 1.5 times thicker than pure-white (office) paper.
Then there is the 'trim size' (i.e. the height and width), the choice of font, the size of the font, the number of images and the overall design of the pages – especially how much 'white space' is used. (White space is the area around the words, and includes the page margins, the space between paragraphs and the space between lines.) All of these will affect the book's thickness.
My general advice is that you want your book to be thick enough that the spine can include a good-sized title and the name of the author. A minimum of one centimetre is a good starting point. A 'perfect' business book, which according to self-publishing guru Dan Poynter comprises 144 pages, will usually be about 10 to 12 mm thick.
Word processor pages ain't book pages
More than once I've sent a client a draft of, say, 90 pages in an MS Word document only to have them come back and say they imagined their book being nearly twice as long. The thing is, an MS Word page will hold (as a ballpark number) 400 words, whereas a typical book page will contain more like 250 words. As you can see, one Word page is equivalent to nearly two book pages.
If you want to get a better sense of the number of pages you have written, try changing the margins in your word processor so that the area you are writing in equates to around 22 cm by 15 cm. Bear in mind this will still only give you a rough guide.
Word count: the real measure of a book
From a writer's perspective, there is only one statistic that counts as you sit down to face that blank screen: the word count.
In the world of business non-fiction, the number of words needed is generally not as onerous as in fiction. Where a typical novel starts at about 70,000 words, a business non-fiction book can contain as few as 25,000 words.
In many ways this is the ideal length: a 25- to 30,000 word manuscript in a typical layout will end up being around that magic 144 pages. It is a length that is easy to consume for the time-poor reader. Better still, it tends to force you, the writer, to be succinct. With only a few exceptions, I would suggest that most of the business books out there that are longer than 35,000 words could probably be shorter without losing anything.