Please outline the steps you are performing to create an empty book, and then the steps to add start text to it.
I created an empty book like so.
1. Visited a page that wasn't in a book.
2. Clicked Options > Add to Book.
3. Clicked + to create a new book.
4. Entered the name 'Empty Book' and submitted.
5. Saved the page to add it to 'Empty Book.'
I then visited 'Empty Book' to confirm it had the default text.
1. 'Empty Book' had the default text.
2. Clicked Edit.
3. Entered some text for the book and submitted.
In this case, my test 'Empty Book' appears to have been updated correctly:
https://www.vaultwiki.org/books/Book:Empty-Book
If you used a different process, please let me know. Otherwise, you will have to submit a Ticket Support service via the Members area so that we can look at your issue more closely.