I recently published a book and was asked to share the first 30 pages so the publisher could edit and send it to the distributor. I complied and asked to see it. When I saw the amount of change the editor had imposed on the work, I was appalled. I had been told she wanted to make some grammatical changes, but she'd added lines of text using a first person pronoun (and the book was a memoir, so all she added was attributable to me, my thoughts) and other stuff I don’t want to go into. For changes like that, I expected some back and forth, or to at least be involved.
I started searching for the answer to this question for myself.
I don't have an agent. I went to a place that offers professional legal advice to artists. The lawyer told me that I didn't have a legal problem, but I had a problem that most often dealt with by an agent. I asked for clues about how to move forward and the lawyer said writers have very little power once the contract is signed. I wonder if that's accurate or if I misunderstood what he was telling me.
Obviously there is a way to exhert control over the publishing process and this is, I assume, the job of the agent. But how does a DIY like me manage?