From Preferences, NoteTaker can be configured to automatically create a back-up version of your notebooks and also be configured to automatically save your work while using NoteTaker.
If the "Retain Backup Files" preference is selected, the backup files can always be found in the same folder as the notebook. The name is the same, but with a "~" inserted, e.g., "MyNotebook~.ntx".
If the Notebook has not yet been manually saved, both the notebook file and its backup can be found in the /tmp/NoteTaker folder. This folder is normally hidden, but can be viewed using the Finder's "Go To Folder" menu item.