Maybe that’s just me but I find the representation of the notebook structure somewhat confusing as is sometimes difficult to tell at first glance which sub-notebook belongs to which notebook especially when there’s lots of layers. Displaying the notebook structure as a tree might help.
Example for reference:
├── Notebook1
│ ├── Notebook11
│ ├── Notebook12
│ ├── Notebook13
│ └── Notebook14
├── Notebook2
└── Notebook3
├── Notebook31
└── Notebook32