For authors considering self-publication, there are a lot of extra roles the author must fill that a traditional publisher provides. Among them is editing. Even if you are going the traditional publishing, route there is the debate of whether or not to use an editor before shopping for and agent or publisher.
SFWA provides a lengthy discussion on when to seek out an editor, what to look for in an editor, and what you will and won’t be getting when you hire an editor. Even better, they provide a few links to well established editors.
May the Muse be with you,