There are many ways for an author to measure success and I'm going to skip all the obvious answers like health and happiness, and stick with publishing industry measures. But I'm going to put a bit of a twist on it, by talking about self publishing industry measures, which have little to do with metrics employed by large trade publishers. Also, I'm aware that it's a bit rude to dictate to authors how they should measure their own achievements, but it's intended as a positive follow-on to yesterday's somewhat discouraging post about authors publishing fiction through subsidy presses or their own company.
Anybody who works at a large trade publisher and tells you they are satisfied when a book sells 5,000 copies is fibbing. Odds are, they aren't even breaking even. Yet, by self publishing standards, 5,000 copies sold is quite a success, and should represent somewhere on the order of $30,000 to $50,000 of net income to an author who is self publishing through his own company. Obviously, the exact amount depends on the costs and the cover price, but it's not chump change. That same author, had the book been published by a large trade and sold 5,000 copies, would probably end up with just $5,000 to $10,000, unless a large advance was involved. As an author who has been published by one of the world's largest trade publishers, with well over 100,000 books sold, I can honestly tell you I wish that I'd self published those books and managed just a quarter of the sales:-)
When you are a working author publishing books for a living, success is measured by the bottom line, and the bottom line is measured in dollars, not in copies sold. That's why relatively few nonfiction self publishers will sell out to the large trades when solicited. We simply make better money self publishing. But for fiction writers the expectations are different at both ends of the scale. For my money, anybody who self publishes a novel and is able to sell a hundred copies, outside of family and friends, is really off to a good start. Anybody who sells a thousand copies of a novel has a self publishing success story on their hands and should probably make another round of the agents and the trade publishers. The large trade publishers really do have an advantage when it comes to selling fiction, so getting published by a name publisher is a reasonable measure of success for the fiction writer.
Another way for authors to measure success is by comparing their books with competing titles or similar genre books by authors in similar circumstances. If you've written a novel and published through a subsidy press with over ten thousand titles in print, and your novel ranks in their top 100 for sales, I'd call that a success, even if the total number of copies sold in relatively small and the project is a financial loss. You shouldn't be comparing your sales to J.K. Rowling or Stephen King, or even to working self publishers for that matter. If you write nonfiction and you find people contacting you as a recognized expert in the field thanks to the book you've written, that's also a form of success. If acquisitions editors start contacting you out of the blue to make offers on your existing books or to ask you to write a new book for their imprint, that also counts for something.
Setting a reasonable goals for book sales is the best way to start achieving those goals. An author who believes that success requires thousands of sales a year is likely to give up in disgust after a month of hard marketing work only sells a dozen copies. An author with the reasonable goal of getting to a hundred copies sold and seeing what happens from there could be satisfied with a twelve books sold that first month. After all, it's just the first month, and the book is already on pace to sell over 140 copies in the year.
Getting book sales is not about asking people or chain buyers to buy your book. It's about getting people to ask you if you have a book for sale. It requires that you establish yourself as a person who either has something valuable to say or a particularly nice way of saying it. That happens outside of bookstores and bookselling circles. For some successful authors, it's all about public speaking, for me, it's all about publishing online. Whatever your platform, you'll have a much better chance of success if you're already standing on it before you write and publish a book. What's more, the view from that platform will help inform you what you should be writing about and who you should be writing for if you want the best chance of commercial success.