With the revolution in self-publishing, the world is getting to be awash in books that never went through a traditional publishing house. It's liberating for writers who labor for years, first crafting a story and then shopping it around to third parties who have no particular interest in seeing you succeed. But there is also a danger, that having bypassed a lot of the usual procedures and filters, you'll publish something that isn't your best work and doesn't make you proud. If you can afford to pay an editor, by all means do so, but make sure the person has a track record. Try not to be that person's first customer. Before you do that, you'll do well to make the manuscript as clean as you can make it on your own. Here are a few of the things any author should do:
1. Read your manuscript to yourself aloud. By forcing yourself to notice every word, you will notice errors and repetitions a quick scan would miss. Make changes as you go along.
2. Run a spelling and grammar check. This doesn't mean you have to accept everything your word-processing program suggests, but it forces you to look at everything in detail and make decisions. You're dealing with a dumb computer that will make a few stupid suggestions along with all the good ones, so keep a dictionary handy and use it.
3. Ask several other writers to read and critique your work. Accept only the suggestions that make sense to you. Never forget that it's still your book.
4. Use a template. CreateSpace, for example, offers free downloadable templates that provide the proper margins and other settings based on the physical size of the book.
Blog Book Tours (firstname.lastname@example.org) will offer free classes soon and will go into a lot more detail. Meanwhile, let me know your specific questions and I'll do my best to help.