An Extra Pair of Eyes
There is an expression in proofreading that involves finding “an extra pair of eyes" to look at your work. If you can afford it, hire a professional to look at your book and give it a proper critique. These type services can be incredibly expensive, though.
An alternative suggestion would be to see if you or any of your friends know an English teacher that might be willing to look it over for a small fee or even as a favor. You’d be surprised how many people are willing to read it over for free just for the honor of being one of the first to read a book. Be sure to offer them an editor’s credit in the liner notes. Restaurant gift cards are greatly helpful, too.
Having someone else look at your writing is crucial because they will catch mistakes that you miss. One problem in writing is that you become too familiar with your own work, and while you think that you might be proofreading something, the reality is that you will skim right over some blaring mistakes.
In my personal experience, I’ve found that for every time I read over something again, I am less likely to catch something that I missed the first time. What happens is you start to memorize passages of text and your mind recalls those passages from memory more quickly than your eyes read and process the words from the page. When this happens, you literally do not see the errors even though you read right over them.