I have proofed/edited several books. The daughter of one of my friends writes romance novels, and I have helped her by reading them for grammar, spelling, punctuation and light content corrections. I don't do significant plot line or storyline editing. I found it is a totally different kind of reading, and having to concentrate on looking at each word and sentence structure takes concentration, but comes fairly natural for me. But, I don't think would do it as a fullime job!