Las Cruces cacti in bloom
Planning to self-publish*? Always hire an editor—yes, I'd like that. I'm an editor. Hire me. Chances are you won't, of course. Chances are you'll hire no one, and that will be a pity for your book's sake.
Here's the deal, though. You must have someone read your manuscript who is not a friend or relative, someone with critical skills whose first interest is not to make you happy. If you belong to a writers' group, ask for critiques of your work, and be prepared to reciprocate. You might consider the Internet Writing Workshop, a helpful group I've been with for years.
Skill levels vary, of course, but getting critiques is an essential start. Don't take everyone's advice, because people can and do contradict each other; critiquers can also be flat wrong or unconstructive.
Once you've compiled and incorporated the best comments from the critiques, take the steps I outlined in my previous post. This should not take the place of hiring an editor, but not being born yesterday, I know that many people either can't or won't spend the money. In the first place, editors vary in quality as widely as writers do. In the second place, many charge by the hour and expect—get this—to be paid a living wage. Soon I'll have a post containing tips for choosing an editor.
Meanwhile, the higher quality your work is to start with, the less work an editor needs to do, and the less the impact on your wallet.
*By self-publishing I mean causing your book to be published, whether by sending your manuscript to a printer or to an outfit such as iUniverse or Xlibris.