Editing is not one thing. Three different jobs get called "editing", and picking the wrong one wastes money or leaves problems in the book. Here is how to tell them apart.
Proofread: fix the small mistakes
A proofread fixes spelling, commas, doubled words and typos, and nothing else. Your sentences stay exactly as you wrote them. Choose this if the book already reads the way you want and you only need a clean copy.
Polish: smoother sentences, same voice
A polish does everything a proofread does and also smooths clumsy sentences and trims repeated words. It should keep your words, your voice and your facts. Choose this if the book is right but a few lines trip you up when you read them out loud.
Big-picture: what is slow, confusing or missing
A big-picture read does not change your text. It tells you, in writing, where the book drags, where a reader gets lost, what is missing and where names or dates do not match. Choose this when you are not sure the book works as a whole.
Which order to do them in
Big-picture first, because fixing structure changes the text. Polish next. Proofread last, because every earlier change can add new typos. If you can only afford one, a proofread is the safest.
What a computer editor cannot do
It does not know your family, your town or your intent. It can miss a wrong fact that reads smoothly. Read the changes before you accept them, and have a person you trust read the final book.
Try it on your own pages
EditPublishSell shows you a free sample of your own words before you pay, with every change marked.
Add your book, choose what you want done, and see a free sample of your own pages.
Start your book
This guide is general information, not legal advice. Store rules change, so check each store's own help pages.