To hire, or not to hire, a professional editor? Here's what Emily Suess has to say about it in her Steps in Self-Publishing post on She Writes:
Your book must be edited by a professional editor. Preferably one who specializes in fiction if you're writing a novel, or non-fiction, if you're writing a self-help or how-to book. In an ideal world, you'll spend most of your money here. Editing and proofing are needed at a couple of different stages. This isn't a once-and-done endeavor. You might need to go through either or both types of editing more than once to ensure that your book is truly ready for publication. Substantive and developmental editing deal with the big-picture questions. Does everything in the book support the overall goal? Is the content engaging for the reader? Are things like plot, character development and dialogue the best they can be?
Copy editing or proofreading are for getting the details just right. In this phase, your editor will help you deal with word choice, grammar, punctuation, typos and spelling errors. If you skip this, people will notice. Often after substantial revisions are made, another proofread is necessary to clean up any straggling errors.