When you get that email, particularly if it's your first book, make sure you are well-rested and in a positive frame of mind before you open it. If it’s your first book, the editing process will be hard on your ego, but remember, the editor’s job is to make your book better and help you learn the craft. You are paying for them to give you critical feedback, not to pat you on the back and say ‘good job.'
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.