How do I edit a whole book without losing the author's voice?

You already know how to fix sentences. Shaping a whole book is a different job, and it asks you to hold the author's voice in one hand while you move big pieces with the other. That tension is not a sign you are failing. It is the actual work, and it is hard for everyone who does it well. The trap is treating the book like your own draft. It isn't. Your job is to help this writer say what they were reaching for, not what you would have written. When a change makes the prose cleaner but quieter, ask whether you traded away something true. Often the rougher line is the one that sounds like them. Start with one chapter. Before you touch it, write a sentence or two describing what makes this author's voice theirs. Keep it beside you as you edit. When a cut tempts you, check it against that note. You will lose the author far less often when you know exactly who you are protecting.