Let’s assume that now you’ve delivered your revised manuscript, the argument/theme/plot/structure/characters have been sorted adequately and the über-editor is satisfied. (If not, just repeat the last stage ad nauseam, or until you decide it’s too much aggro and you don’t want to publish the book, or until your editor goes on maternity leave – they all do eventually – and the next editor can’t be arsed and either lets the book through or cancels it.)
Time to move on to the next editorial stage, copy editingr. A copy editor deals with the detail of the words on the page – the ‘copy’ - turning them into ‘book words’ as they have become known, post-Jordan. If the book is largely text (such as a novel) changes will be intended to make the book read well, be grammatically correct and consistent. If the book has pictures, whether it is fiction or non-fiction, copy fitting is also involved. This aims to fit the copy to the spaces on the page allocated to text. This can be very frustrating, as material you consider useful – even essential – may be cut, or clumsily extended, in the process of getting the right number of words on the page, in the right text boxes.
If you have written well and know your audience and market, not much will change, but the book may still have to be cut to length. A good, considerate copy editor will preserve your personal style and will improve the text at this stage. If you are lucky, (s)he will have Word’s Track Changes option turned on so that you can see what has been done. (I presume you know about Track Changes? If not, shout now and I’ll tell you about it.) Where more significant or extensive changes are needed, the (good) copy editor will ask you to write the new text, detailing what is required. The editors comments and requests may be in Word Comments, in square brackets or, more usually, in a different coloured text (and square brackets). If it is a factual book, there may be queries about facts at this point. There may even be an expert reader (consultant) who can find mistakes for you to correct, but that depends on how wealthy your publisher is and how seriously they are taking the book (and how much they trust you to get it right, I guess).
Bad copy editors will ride rough-shod over your text apparently making changes just for the hell of it, either to justify their fee or to make their mark on the text. They will mangle your prose to the point where an A&E doctor would put a Do Not Resuscitate notice over it and order a body bag. Bad copy editors introduce grammatical errors, factual errors, anachronisms, inconsistencies… they buy them in bulk at Tesco and sprinkle them liberally. You will be lucky if you find them all before the book goes to repro.
Editors should have a good grasp of the English language. Sometimes they have too good a grasp of the English language and are reluctant to loosen their vice-like grip and allow the language any freedom to play. In this case, they will contort sentences with their (strictly speaking) accurate use of ‘whom’ and ‘whilst’ that is entirely unsuitable for a six-year-old reader. Medievalist Malcolm Parkes once told me he wanted to write a book on punctuation called The Strangulated Colon. He didn't intend it to be about these editors, but it could well have been. Some editors have only a slippery grip and the language eludes them, leaving the text strewn with split infinitives and hanging prepositions (fine if they’re yours and you chose them – infuriating if they have been foisted upon you). Worst (in my view) are those editors who cut a text to length by apparently taking out half the words in each sentence, leaving some half-thing crippled by terminal parataxis.
To correct the ‘corrections’ of a bad editor you need to explain exactly what is wrong with the revised text in a way which brooks no argument - not by being stoppy, but by being incontrovertibly correct. You need a well-honed critical capacity and the right vocabulary. It is not for the faint-hearted, but it’s worth it – it’s your text. Very occasionally, you may need to appeal to a higher authority within the publishing house. If you feel they have really mangled your text beyond the point where it could be identified even by dental records, you can plead violation of your moral rights and withdraw it - or ask for your name to be removed, if it was a commissioned piece and you want to keep/have already spent the money. More on moral rights another time.
With a picture book text, in which every word counts, I generally explain each and every change that I want to make to the editor’s version of the text. You are far more likely to get it changed if you can support your case cogently. Refer to the cadence, legibility, vocabulary/age match, ease of reading, repetition – any objective criteria you can think of that explain why your way is better. If you can’t explain why your way is better, perhaps it isn’t. Oh, and the point is not to get all your original text restored, but to get the best text. Thank the editor for improvements; acknowledge where they have done a good job. And if they didn't like your text and you don't like theirs, perhaps you can find a new way of putting it that you will both like.
Whether you should compare the edited text with your original is a matter of opinion. I do in the case of non-fiction because of the accuracy issue, but with fiction I don't do so until I am happy with the edited version and my new changes to it. It's useful to see whether it really is better than the original (it often is), but it's hard to set aside your allegiance to the original if you look at it too early on in the process.
Text can go backwards and forwards several times, accumulating comments and rewrites as it goes. At some stage, when the editor is happy with the text, it will be sent for layout – more about that another. The changing around hasn’t finished yet – there may well be more checking and changing to do after layout.
Ultimately, your aim should be to work with the editor to produce the best book possible. The editor is not your enemy. You are both working towards the same ends. Most editors are good, clever people who can do their jobs well, and if they sometimes make mistakes, well – so do you. The relationship of author to editor is rather like that of parent to teacher. To you, your book/baby is the most special in the world and can do no wrong; to the editor, it is one of many to be dealt with and, good or bad, could probably be made at least a little more presentable and be encouraged to fit in a little better with the rest of the list/class.