Many writers believe that self-editing their manuscript or having a friend or family member review it is enough prior to publication. It’s usually not. Without professional editing, your manuscript will likely contain several grammatical errors and maybe even some major plot holes, structural issues, or character inconsistencies that you’ve overlooked. It's easy to become blind to your mistakes when you've been working on something for so long and are too close to the material.
Professional book editors understand editing standards, genre expectations, and industry trends, and can be hired at any point in the writing process to help strengthen your manuscript. If you've submitted your manuscript to agents or publishers and have received rejections or non-responses, you should consider hiring an editor to do an assessment. He or she can help you determine what's holding your story back from getting a publishing deal. For writers that are pursuing a self or independent publishing path, consulting a professional editor prior to publication will greatly increase your chances of producing a polished book that will attract readers. In this collection of interviews, authors and editors discuss the benefits of working with a professional editor and what to expect with that partnership.
I find this all interesting as I will find out how the first book goes, it's a learning experience we will see!