It can be tempting to ask a friend to edit your book or someone who is not an editor but who is good with language and is prepared to do the job for little or no cost.
The issue here is thoroughness. Editing is a profession like any other. It is an editor’s job to help the author produce a work that will keep the reader engaged and cause that lasting effect the author sets out to achieve.
Still not convinced? Why not give a professional editor a go? Any good editor will offer a free sample edit. Judge the difference in quality for yourself.