Most every author dreams of selling a million copies of their book. Or even a hundred thousand copies. They gauge their success by how many units they've moved, which makes sense because each sale equals a royalty earned, and that's how career authors make their living.
There are some tried and true strategies to selling loads of books. You could be a celebrity--entertainer, famous chef, politician, billionaire entrepreneur. If you already have a huge ready-made audience, you can be pretty confident you will be able to tally up those 6-7 figure sales numbers. You can also have an inspiring personal story of triumph over impossible odds--e.g., a rags to riches tale--or a rousing account of how you achieved some incredible goal--e.g., climbing Mount Everest 100 times--that can serve as an example affecting multitudes of readers.
But if you're like most authors, you haven't (yet) built up legions of fans worldwide, and maybe your motivational memoir or business manual hasn't (yet) ramped up enough sales to hit the NY Times bestseller list.
But that doesn't mean you've failed. Not at all.
When you consider how many books are published yearly--more than 600,000 in the U.S. alone, according to this Forbes article--it's a wonder any author who isn't already a big shot can sell copies beyond only their family and friends. Yet many authors do just that. And you don't have to sell one million copies, or even a hundred thousand, to call your book a success and land on Amazon's bestseller lists.
So if you don't sell a ton of books, how many should you sell to merit celebrating? That depends on what your goals are. Not just for your book, but for your overall career. If you accept the hard truth that you will probably never make a living as an author--don't count on buying that mansion in Malibu--you can then approach your book as something else equally valuable: a marketing tool for your business.
Writing and publishing a book offers its author respect and credibility as an authority in their field. And if it's on a popular or specialized topic (e.g., dieting & fitness, entrepreneurship, money management), the book can easily appear on Amazon's bestseller lists in that category. This in turn gives you more exposure, allowing more people to find your book--and by extension your business--when searching its subject on Amazon. Additionally, you can sell your book or give it away at any trade show or other marketing event your company attends.
Prospective customers are far more likely to trust those they perceive as experts, and experts are far more likely to earn their business. A book creates an image of knowledge and know-how for its author... even if only your parents and best friends have actually bought copies.
Just as you shouldn't judge a book by its cover, do not judge its success by its sales. The lack of revenue your book may generate in royalties could well pale in comparison to the amount of business it generates for your company.