Has your book project been accepted? Congratulations! Now the real work starts.
You should receive a book contract, which will tell you in detail (and maybe unreadable legalese) what rights and obligations you have as an author. Having only seen one German publisher’s contract thus far, I can only speculate about what you might encounter in your document.
The contract will tell you what you agreed to with the publisher: topic and length of the book, the deadline for handing in the final manuscript, the publication date and other things. The publisher will tell you which rights you will transfer to the publisher by signing the contract (copyrights etc.). You will also learn about which fees you will receive and when, how much discount you get on your own book and how many copies you will get for free after publication. The contract may also tell you what the publisher expects you to do in terms of marketing as soon as the book is out. All in all, the contract may encompass several pages. Anything else would surprise me.
Having signed the contract and celebrated the first crucial step in the publication process, it’s time to sit down (again) and write. You’re supposed to hand in your manuscript on time – no excuses. In the process, it may happen that your book’s focus will change or that some aspects, as agreed with your editor, no longer work. Tell your editor as soon as possible. I don’t think that the publisher will see this as a breach of contract or as disrespectful. However, if you only tell them when you submit your manuscript on the day you’re supposed to, you might face consequences. So don’t ruin the experience and do what it is that you signed up to do.