InputNotebook[ ] gives the current notebook into which keyboard input in the front end will be directed.
InputNotebook returns a NotebookObject.
If there is no current input notebook, InputNotebook[ ] will return $Failed.
The current input notebook is the notebook to which textual commands in the front end are normally directed.
A palette window can be a currently selected notebook but cannot normally be an input notebook.
See The Mathematica Book: Section 2.10.3.
See also: Notebooks, SelectedNotebook, EvaluationNotebook, ButtonNotebook.