So, when you try to delete a notebook, this comes up, it doesn't say it will delete all the notes, but you know that it will. ( Common Sense ) :
You press delete, then when you look at the notebooks, it says trash right below it:
Click that, then you can just hold your cursor on "Restore" and keep on clicking until they are all restored:
Or, you could just make a new Notebook, and move the notes from one to the other, then delete one, or just rename the notebook:
You have no Idea how long it takes me to edit these pics to help you. 😊