I recently edited a book for a friend who writes books about overcoming the trauma of sexual abuse. Tough material to deal with, but it needed spelling, grammar and sometimes intent. She is a wonderful person and writer, but didn't have the same English teachers Jan did. Years ago I corrected college English papers for my second daughter. She was a great teacher and writer, but not speller, nor could she identify a run on sentence.
So, yes, I could and have edited a book.