There's no way to bring back a notebook that you deleted, unfortunately. However, in case it's helpful, when you delete a notebook, it doesn't delete the notes or other annotations that were inside the notebook – only the notebook itself is deleted. You may be able to find the notes again and add them to a notebook, by looking in Notes > All, using search, or going to any chapters the annotations are associated with as a highlight. iOS Gospel Library, Android Gospel Library, and Gospel Library Online behave the same when you delete a notebook – the annotations themselves aren't deleted automatically. I'm not sure about Windows Gospel Library.
Samuel Bradshaw • If you desire to serve God, you are called to the work.