I've been doing freelance nonfiction book editing since April '08 and just started my twentieth book.
Yesterday I received my first thank-you, from a co-author of the nineteenth book.
One out of nineteen: It doesn't make it all worthwhile, but it helps a bit.
I've been an editor since '83 and understand that this is a profession in which its practitioners are judged primarily by their mistakes, not by their contributions. I can live with that.
Anyone who needs to be thanked for a job well done needn't apply.