Ted Thompson, author of The Land of Steady Habits and Sackett Street instructor, answers this question on the Little Brown site in six parts, all very useful. Here’s an excerpt:

Selling a book won’t change your life—except it kind of will
OK, here’s a little experiment in radical honesty. I sold my book for $25,000. I also sold some foreign rights, which ended up about tripling that, minus taxes and agent fees. I am proud of this and see it as a huge personal success, though during the years that I worked on the book it would be a lie to say I hadn’t maintained a vague idea that if I could just finish the thing and sell it that it would relieve my money worries and set me on a path to financial stability. I think this is something I had to tell myself, that there was a promised land just over the horizon, so I should keep going. And while it feels overly cynical to say that for a writer financial concerns will never go away, here’s what I’ve come to realize: for a writer financial concerns will never go away. There are many exceptions to the rule, of course, but most of what we do is trade the time that we could be making money for the time to write. Sometimes those two equal out, but for most people they don’t. So you have to figure out how you define success. Is it readership? Is it recognition? Is it in how much you produce? Or in how you feel about your own work? I’m still negotiating this with myself and probably will be forever, but one thing I have noticed is that having the book out in public has already changed my relationship to writing, and not in the ways that I thought it would. I worried that I would become fixated on how the next thing would be received, bringing everyone in publishing and the wider reading world into the room with me, but it has actually made the process more private, more internal. It’s no longer about clearing some imaginary bar of professionalism, no longer about gaining entry into some club. I’ve talked about this with a friend of mine who’s a musician, but putting something out in the world and listening to the cacophony of reactions can actually have the effect of releasing you from what you had imagined others wanted, and in a way giving you back your space. This, for me, has been the healthiest outcome of publishing a book, and something I don’t think I would have gotten without it. Permission, I guess, to no longer ask for permission.