At first, success seems to be based on certain objective metrics.
When I self-published my first book in January 2011, I thought success to be tied to book sales. I soon found out (by selling a grand total of 2 eBook copies and 2 paperback copies over the next 6 months) that success can get quite confusing, especially if you're just starting out.
That's what we're going to explore throughout this guide.
How do we define a self-published book's success? How do book sales factor in? What really matters, especially if you've just self-published your first book?
The Measure of a Book’s Success
I’m going to tell you something very counterintuitive:
Book sales are not a good way to measure the success of a book.
More and more authors today are finding success through focus—reaching just a few people in a profound way—because those readers are far more likely to do something with it.
Authors who self-publish are free to use their book however they want: generating revenue through speaking, coaching, and consulting; or giving the book away just for the joy of knowing they’ve made a difference.
Traditional publishing will not let you do that. Publishers only make money by selling copies of books, which is why they obsess over book sales.
But self-published authors have many more roads to success.
Instead of asking how many books you should expect to sell, you are better served by asking yourself how many opportunities, how much revenue, and the amount of personal fulfillment you can generate by putting the right book in front of the right people.
Don’t get me wrong: it’s great to sell books. It’s just that focusing on book sales often trades off with other forms of success, so you have to be clear about what you prefer.
Identity
Self-publishing my first novel was the side-effect of taking part in NaNoWriMo. The logic behind this was that once I had a physical book (with my name on the cover!) in my hands, I'd double down on writing and finally be able to call myself an author.
If you're new to the self-publishing game, that's what I recommend that you focus on with your first book.
Use your first book to change your identify from that of a writer to that of an author.
This might seem like a subtle mindset shift, but it does matter an awful lot, especially once you realize that you will have trouble measuring your book's success based on book sales alone.

The endgame is simple: publishing a book will enable you to call yourself an author (and in and of itself is a measure of success as a writer.)
Congratulate yourself for turning the blank screen of a Word document into an actual book.
Impact

Another important aspect to consider when measuring your self-published book's success is the impact it has on those that read it.
It does not matter if you sell 2, 25, or 2,000 copies, all that matters is the way people react to your book.
This, of course, can be as simple as keeping track of reviews, or printing out the emails you receive from those who read your book.
I recommend that you focus on this step because commercial success as a self-publisher is far from guaranteed, and you will be successful at breaking your own heart if you focus solely on the cold perspective offered by sales numbers.
Growth

The third step I recommend you take when measuring your self-published book's success is growth.
Are the numbers growing?
Are book sales growing?
Are you developing new skills, becoming better at marketing your self-published book, and being more effective at turning a blank document into a finished first draft?
A simple equation to keep in mind is this:
PROGRESS = SUCCESS
As long as you feel like you are selling more and more books, receiving more and more reviews, and learning and improving your skills as a indie writer, you will also feel like you are a successful self-publisher.
My advice is this: if you want to feel like you are actually progressing with your self-publishing, you need to take massive action. Turn all the tasks that you feel you should do and turn them into tasks you must do.
You've already shifted your identity from that of a writer to that of an author, you already know that your book has a positive impact on your readers, now it's time to take massive action towards ensuring that more and more people benefit from your book.
This requires that, yes, you deploy certain strategies in order to sell more books, but it also requires that you develop a new set of skills, so you can better promote your self-published titles.
Now, understand that it's not healthy to measure your book's success against the results of other authors within your niche, which is what most authors do regardless.
Instead, try to understand that your book's success is tied to the way it changes your identity as a creator, the way it positively impacts others, and the actions you take toward progress.