Teachers, friends, and family members do not always make the best editors. You need an unbiased eye on your work -- someone who is more interested in crafting good sentences than in saying nice things about your work. Your Great-aunt Zelda may be willing to struggle through a poorly written book, but the average unknown reader being asked to plunk down $23.95 for your book may not be so patient or forgiving of poor writing.
Every book, even those written by Nobel Prize winners, needs editing and proofreading. These are two different processes and have two distinct purposes.
Editing is more time consuming than proofreading, and so, it usually costs more.
The Importance of a Professional Eye. At some point -- either editing or proofreading -- you need to have a professional look at your work. A book is far different from a term paper. It's written in a different manner and for a different audience. An editorial professional will know what is acceptable book style and will focus on creating a book that is well-written, tightly woven, and accessible to readers.
Won't my publisher edit my book? If you are not self-publishing and hoping to find a publisher for your work, you still need a professional edit. Many publishing houses have cut back on staff. They don't have as many editors, and so, the few they do have want to see polished work. If your work is likely to require too much editorial attention, the publisher may pass on it. So get your work in the best shape it can be -- before you submit it to a publisher.