At SLPA we tell you to get an editor for your book. No, not your sister or even your child's language arts teacher. These people can be your first-line editors, but you need someone who's great at grammar, spelling, and reading critically. It does cost some money to hire a good editor, but at the least, hire a copyeditor to take care of the basics so you don't embarass yourself or contribute to the stereotype of the awful self-published book. Always ask for a sample edit of the first chapter or the first few pages to see if your editor understands your style and concept and to see how good they are. If you don't know much about grammar and sentence structure, get someone who does to look at the sample - anyone can say they are an editor, but are they any good? Below is a link to an article on editing by Daily Writing Tips, a good blog to follow.
How to Hire an Editor