There is more to editing a book than making sure that the spelling, grammar and punctuation is correct, essential as that is. Some manuscripts need just a little light polishing — but some need more. Is the best opening chapter buried deeper in the text? Is the narrative too linear, too simple? Is it too complex?
If this is fiction, are there too many characters? Are their actions consistent with their personalities? Are their voices clearly distinct? Is there a strong subplot? Are there too many subplots? Is an essential scene missing, or even a minor one that would give a change of pace or clarify motivation?
If this is non-fiction, does the structure convey the information clearly? Is the vocabulary clear and jargon-free? Is there any unnecessary repetition? Is there any information missing? Is there a likely market for the book? If there is, does the book present the information in a way that will appeal to that market?
Then there is fact-checking: I do as much of this in fiction as in non-fiction. For example, I have used Google Street View to check that a line of sight in a crime novel worked. (It didn’t.)
With every book, whatever the subject or genre, I will gently challenge the author to make sure that all elements work together to make the book the best it can be.