Over the years, I've been asked to edit other authors' books. Sometimes I'm asked to content edit, the broadest type of editing, and sometimes to line edit, which - as you've probably guessed - takes the editing process to a minute level.
There are things I've learned about editing another author's works - or even giving my opinion:
1. Never try to change another author's vision.
What might take me a few hours to read might have taken another author months - or even years - to write. Their book is something they have lived with: the plot, the characters, the way the story unfolds. Though some suggestions can be made to make the book better, it should never attempt to change the story they want to tell.
2. Never try to change the author's voice.
You can point out changes in point of view, but you should never try to alter the way they communicate a story. Every author's voice should be uniquely his or her own.
3. Never pass judgment.
The book might not be your cup of tea but that doesn't mean it doesn't have merit. Just google "famous bad reviews" to see the truly horrendous things that editors have made about books they didn't like - books that often went on to sell millions of copies.
4. Avoid absolutes.
Never say "it HAS to" or "never, ever, EVER do this"... There are no absolutes in this business. An example: in Charles Frazier's Cold Mountain, the author uses a dash at the beginning of text in lieu of open quotes usually encountered during conversations.
5. Never dash another's hopes.
Before providing criticism or a critique, ask yourself what your objective is and how your words could impact the author. The end result should always be positive. If it's negative, it says more about you than it does the author.