Of all the parts of self-publishing that authors should get help with, editing will be your first priority. You don’t have to pay a professional editor, but you do need your book looked at by someone who can be dispassionate and knows their way around a style guide such as the Chicago Manual of Style or the AP Stylebook. This well-written article explores the common myths and the key truths about book editing and editors. Use a wise eye and a cool head to glean what it offers.
Effective copy editing removes verbal noise from your manuscript. Great copy editing is invisible and makes you look like a better author. Here’s a good article self-publishers on the promise and pitfalls of doing your own copy editing. Summary? Proceed with caution.
Use the AP Style Book as your general-purpose copy editing resource, online and in book form. I also have the Chicago Manual of Style handy for a second opinion. And there’s lots of help online. Search “English grammar and style.”