Line editing is begun after the author has finished entering all the changes from the content editing, has checked the manuscript for errors and has had it formatted for publishing. The line editor's job is to look for typos, grammatical errors, formatting mistakes, etc. No matter how careful the author and formatter may have been, the line editor will still find plenty of mistakes. This is because of how the human brain works: when we become too familiar with something, we just don't see the errors anymore. We see what we "know" is there instead of what's actually in front of our eyes.
This final step begins after the author has
finished entering all the corrections from
the line editing. Proofreading is the last
chance to catch any embarrassing errors
In general, I follow the Chicago Manual of
Style. But I believe communication always
trumps rules and the author's personal
style is an essential part of that communi-
Every book should go through at least three phases of editing, not including the author's work of writing multiple drafts and checking the manuscript for errors:
The content editor works with the author to identify plot holes and confusing passages, improve character development, tighten suspense and so forth. Often, what we see in our heads and what we actually write down are two different things, but our brains fool us into not being able to see the difference. Part of the content editor's job is to help the author communicate their vision to the readers. Another part is to bring up possible changes that readers may prefer to see.