As an editor and as someone who enters in corrections from editors into books, I would have thought I'd be prepared for this stage of the process. My editor (see how seamlessly I fit that in) wrote a few new paragraphs and left almost no sentence unchanged. Luckily for me, and for him, this book is not my baby so I'm less defensive than the average author.
I figured he would edit for grammar, clarity, and consistency but was surprised to find entire sentences using words I had never heard of nor knew the meaning to. Most of the editors I work with don't add in complete thoughts but just ask the author to clarify or suggest adding in a paragraph.
Thanks to advice from other bloggers, I sat on the corrections for a few days before responding. I realized that a lot of the new paragraphs were mine, they were just cut and pasted from different places and tweaked. I also realized most of his changes were more nitpicky than actually changing of meaning so I let those in. The only changes I really objected to was when he added sentences that I felt missed the tone of the book. Those I edited to be more my voice.
All in all, it was good first step on the road to being a published author.